TYPO3 Core

Couldn't write to https://forge.typo3.org/issues/82093 because this issue was closed:

Johannes Hollmann mentioned that this fix opens another bug (https://forge.typo3.org/issues/82093#note-8).

My suggestion is to change the \TYPO3\CMS\Fluid\ViewHelpers\Form\AbstractFormFieldViewHelper::getMappingResultsForProperty method.
This method accesses the $originalRequestMappingResults. Because it enforces a hierarchy (first level is the form name, second level the field name), the mapping result creates empty results for the requested path (\TYPO3\CMS\Extbase\Error\Result::forProperty).
It should rather access the fields on the root level (without the form name level).
As this happens in a controller context and the properties are already mapped by \TYPO3\CMS\Form\Domain\Runtime\FormRuntime::mapAndValidatePage there might not be any reason to have this hierarchy.

However I don't know how this behaves when there are multiple forms on one page.