When the DelegatingValidationListener is refactored (see #3903), this can be implemented. What you want is basically a custom mapping function that maps the property paths

data[0]
data[1]

to the forms

children[Mon].children[0]
children[Mon].children[1]

A second solution for your problem is to make the forms Mon, Tue etc. virtual. Virtual forms are used for presentation only and are not mapped directly to the domain model, resulting in the mapping:

data[0] <-> children[Mon].children[0]
data[1] <-> children[Mon].children[1]
data[2] <-> children[Tue].children[2]

To do this, you would have to move your value transformer logic to a form listener that rebuilds the form every time your data is set.

Thanks for your explanation. For now I'll keep it as-is and wait for #3903 to be resolved. The form between the base and the entity forms also contain quite some logic which would probably result in a lot of refactoring and possibly unclean code. I'll resort to the virtual solution if I'll find myself running out of time before the issue is resolved

Can you please verify if this is fixed by #4341?

@bschussek it appears the bug still exists. The error is still mapped to "data[2].break" rather than "data[Wed][0].break" (or something similar)

Anything I can do to help you debug?

I think that the mapping function I described above is not really feasible. I rather think now that your use case is exactly the use case of virtual forms.

Could you test whether virtual forms fix your problem?

@bschussek When the deadline approached a while ago, I implemented the work-a-round you provided around a month ago, and this indeed worked fine.

Does this mean that the way the form was before, is actually not intended use?

Yes, forms are not intended to be used like that. A form that does not map to a property of the underlying data should either be unmapped ("property_path" => false, or "mapped" => false as of #4341) or virtual ("virtual" => true).

See also symfony/symfony-docs#1387