Possible documentation issue with forms in modular pages

The documentation on forms in modular pages says Form v2.0 is fully backwards compatible, this might not be true with certain templates.
I have a website with a form in a modular page. My theme is inherited from quark. After an update from 1.7.41 to 1.7.46 the form simply disappeared without any obvious error. The page pretty much followed the documentation when it was made. After trying to move the form definition from the modular.md to the modular sub-page the problem disappeared. This behaviour happened with the quark theme but not with the deliver theme. When the error ocurred, the warning "Form unnamed not found in page /contact/_contact" was cast.