Not sure if this is a new bug introduced in the current version (21.2.2, Build 465), or possibly specific to iOS?
On an android device running 21.2.1, the edit process seems to work.
On multiple iOS devices, on the current build I am able to edit a features attributes, however if I try to edit a related record, I tap 'Submit', I get a Submitted confirmation, however the attribute changes aren't applied to the related record.
I'm trying to find an iOS device on the previous version to check
Another complexity is that the related table is using Subtypes to allow a type/subtype hierarchy.