I did a lot more testing and updated the issues.
The bug causing crashes on older OS if build with Xcode 16.1 or 16.2 has been narrowed down somewhere between RealmSwift 10.38.3 and 10.40.1.
I tested forward from 10.14.0 (my current shipping version) on an iOS 14 device. Xcode16 cannot debug on my older iOS12 and 13 devices.