Hi Nicholas,
I have also come across a similar situation. The solution which I would suggest for the mentioned case is to encapsulate the Form within the block and refer the same inside the popup.
See this sample app with the same implementation.
Check this Forum post: https://www.outsystems.com/forums/discussion/59495/mobile-app-validation-message-not-showing/#Post227181
Hope this helps you!
Kind regards,
Benjith Sam