I reverted (locally) changes from #28966 and I'm trying to fix it differently.

@xabbuh indeed, the problem is in getWriteAccessInfo()
Instead of checking whether the property itself is plural or single, it checks if the passed $value an array:

The problem is that when isWritable is called - it doesn't have $value and an empty array is passed, just as a stub.

I added a check if the property name plural. So it uses adder/remover only if the property name is actual plural.

That is - $camelized preserves its original form (plural of singular), $singluars doesn't have plural by design.
So if $singulars contain the original passed value - we 100% sure the singular is meant here. And vice versa. So we can't use it for adder/remover because plural is required here.
Tests pass locally.

@xabbuh @nicolas-grekas makes sense?

my idea to fix the issue would be #29355

@xabbuh I see - it shouldn't cache the getWriteAccessInfo result without taking into account $value.
But still it doesn't distinguish, for example, isPropertyWritable('email') and isPropertyWritable('emails') they both will be calling getWriteAccessInfo with an empty array argument. I think that the property name should tell whether it's plural or singular.

What do you think of this approach?

$plural = empty(array_intersect(array($camelized), $singulars));
$useAdderAndRemover = $plural && (\is_array($value) || $value instanceof \Traversable)

Closing in favor of #29355, thanks for your help!