Entire array Field rerenders when a property on any array element is mutated (regression in 1.27.0) #1925

Describe the bug

Starting from tag 1.27.0, when working with a form.Field that uses mode=array, when any child Field under the array Field is changed (resulting in a modification to a property on one of the array's elements, not to the ordering/size of the array), the entire array Field rerenders.

This is not what happened prior to 1.27.0; in older releases, only the child Field that was touched would re-render (along with anything subscribing to that Field's backing data), as one would expect.

I noticed this in my own application but it even happens in the simple demo on the docs site: https://tanstack.com/form/latest/docs/framework/react/examples/array?panel=sandbox

The result is that in large complex forms that utilize array fields to show many editable/reorderable field groups, performance is quite poor when users type into any field.

Your minimal, reproducible example

https://codesandbox.io/p/devbox/github/tanstack/form/tree/main/examples/react/array?embed=1&theme=dark

Steps to reproduce

1. Open devtools and enable rerender highlighting
2. Click "Add Person" three times
3. Focus Name field for middle person
4. Enter a few characters
5. Observe the entire array field rerenders, not just the single name field being modified

• The profiler will report "Hook 5 changed" which appears to be the useSyncExternalStore containing the entire array. Array identity is changing?

Expected behavior

When modifying a child form.Field mounted under a form.Field of mode="array", only the child field should re-render when it changes. The entire array form.Field container should not re-render.

How often does this bug happen?

Every time

Screenshots or Videos

Platform

• OS: Linux
• Browser: Firefox 145.0.1
• React: 19.2.0, 19.2.1

TanStack Form adapter

react-form

TanStack Form version

Observed in 1.27.0 / 1.27.2

Does not occur in 1.26.0

TypeScript version

No response

Additional context

• Does not seem to depend on...
  • whether the form uses a zod/standard schema validator
  • whether onBlur vs onChange validators are used
  • whether the array Field is using withFieldGroup or withForm composition
  • whether React Compiler is being used

I am using React Compiler in my application. However the demo application on the Tanstack Form docs site does not seem to be using Compiler and exhibits the same behavior.