diff --git a/src/utils.ts b/src/utils.ts
index a5869870..6bb03f36 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -113,7 +113,7 @@ function warnAboutNewConditionalOptions(newOptions: unknown[]): void {
  * @param schema2 - The conditional branch schema to merge from
  * @param options - The form options
  */
-export function mergeSchemaBranch<T extends Record<string, any>>(schema1?: T, schema2?: T, options?: CreateHeadlessFormOptions): void {
+export function mergeSchemaBranch<T extends Record<string, any>>(schema1?: T, schema2?: T, options?: CreateHeadlessFormOptions, insidePropertyMap: boolean = false): void {
   // Handle null/undefined values
   if (!schema1 || !schema2) {
     return
@@ -135,6 +135,14 @@ export function mergeSchemaBranch<T extends Record<string, any>>(schema1?: T, sc
 
     const schema1Value = schema1[key]
 
+    // A `false` property subschema is unsatisfiable, so `allOf` semantics keep the property
+    // forbidden no matter what a sibling branch adds; otherwise a branch object overwrites the
+    // `false` and the property wrongly becomes valid again. Scoped to property subschemas so a
+    // branch can still relax a boolean keyword like `additionalProperties: false`.
+    if (insidePropertyMap && schema1Value === false) {
+      continue
+    }
+
     if (isOptionsLikeSchema(key, schema2Value)) {
       // Restrict option-like arrays to the options already present on the base field
       if (disallowNewConditionalOptions) {
@@ -171,7 +179,9 @@ export function mergeSchemaBranch<T extends Record<string, any>>(schema1?: T, sc
     if (isObject(schema2Value)) {
       // If both schemas have this key and it's an object, merge recursively
       if (isObject(schema1Value)) {
-        mergeSchemaBranch(schema1Value, schema2Value, options)
+        // The direct children of `properties`/`patternProperties` are property subschemas, where a
+        // `false` value forbids the property and must survive a sibling branch.
+        mergeSchemaBranch(schema1Value, schema2Value, options, key === 'properties' || key === 'patternProperties')
       }
       // Otherwise, if the value is different, just assign it
       else if (schema1Value !== schema2Value) {
diff --git a/test/fields/visibility.test.ts b/test/fields/visibility.test.ts
index b3915d1a..5f213369 100644
--- a/test/fields/visibility.test.ts
+++ b/test/fields/visibility.test.ts
@@ -744,4 +744,51 @@ describe('Field visibility', () => {
       })
     })
   })
+
+  describe('a forbidden property with a sibling branch constraining the same field', () => {
+    const schema: JsfObjectSchema = {
+      type: 'object',
+      properties: {
+        contract_duration_type: {
+          type: 'string',
+          oneOf: [{ const: 'indefinite' }, { const: 'fixed_term' }],
+        },
+        months: {
+          type: 'number',
+          minimum: 1,
+        },
+      },
+      required: ['contract_duration_type'],
+      allOf: [
+        {
+          if: {
+            properties: { contract_duration_type: { const: 'fixed_term' } },
+            required: ['contract_duration_type'],
+          },
+          then: { required: ['months'] },
+          else: { properties: { months: false } },
+        },
+        {
+          if: true,
+          then: { properties: { months: { maximum: 12 } } },
+        },
+      ],
+    }
+
+    it('hides the forbidden field and rejects a value for it', () => {
+      const form = createHeadlessForm(schema, { initialValues: { contract_duration_type: 'indefinite' } })
+      expect(getField(form.fields, 'months')?.isVisible).toBe(false)
+
+      const { formErrors } = form.handleValidation({ contract_duration_type: 'indefinite', months: 5 })
+      expect(formErrors?.months).toBeDefined()
+    })
+
+    it('shows the field and accepts a value when the branch requires it', () => {
+      const form = createHeadlessForm(schema, { initialValues: { contract_duration_type: 'fixed_term' } })
+      expect(getField(form.fields, 'months')?.isVisible).toBe(true)
+
+      const { formErrors } = form.handleValidation({ contract_duration_type: 'fixed_term', months: 6 })
+      expect(formErrors).toBeUndefined()
+    })
+  })
 })
diff --git a/test/utils.test.ts b/test/utils.test.ts
index e1aeee6a..4a8a09e5 100644
--- a/test/utils.test.ts
+++ b/test/utils.test.ts
@@ -181,6 +181,35 @@ describe('mergeSchemaBranch', () => {
     expect(schema1.meta).toEqual({ nested: true })
   })
 
+  it('should keep a `false` property subschema when a branch adds a constraint', () => {
+    // A `false` subschema is unsatisfiable; per allOf semantics it must survive a
+    // sibling branch that constrains the same property, not be overwritten by it.
+    const schema1: Record<string, any> = { properties: { m: false } }
+    mergeSchemaBranch(schema1, { properties: { m: { maximum: 12 } } })
+    expect(schema1.properties.m).toBe(false)
+  })
+
+  it('should let a `false` branch subschema forbid a previously-typed property', () => {
+    const schema1: Record<string, any> = { properties: { m: { maximum: 12 } } }
+    mergeSchemaBranch(schema1, { properties: { m: false } })
+    expect(schema1.properties.m).toBe(false)
+  })
+
+  it('should keep a `false` property subschema forbidden across repeated branches', () => {
+    const schema1: Record<string, any> = { properties: { m: false } }
+    mergeSchemaBranch(schema1, { properties: { m: { maximum: 12 } } })
+    mergeSchemaBranch(schema1, { properties: { m: { minimum: 1 } } })
+    expect(schema1.properties.m).toBe(false)
+  })
+
+  it('should let a branch relax a boolean keyword such as `additionalProperties`', () => {
+    // The forbidden-property guard applies to property subschemas, not to a boolean keyword
+    // whose `false` a branch may legitimately loosen.
+    const schema1: Record<string, any> = { additionalProperties: false }
+    mergeSchemaBranch(schema1, { additionalProperties: true })
+    expect(schema1.additionalProperties).toBe(true)
+  })
+
   it('should skip if/then/else properties', () => {
     const schema1: Record<string, any> = { type: 'object' }
     mergeSchemaBranch(schema1, {