Were you in expanded mode or compact mode at the time? Compact mode has no way to fit the UI above the keyboard as they take the same space.

iOS 12.5.5 can be installed because it's the highest version for a range of hardware that was dropped from iOS 13 but is still around, eg: iPhone 6, iPod Touch 6. Similar frustration - I just encountered a crash report that's an Apple bug with 14.8 and wanted to restore an old phone to it but can't download.

@JosefPro as I say in my other comment, it's not SwiftData! This is a general problem with FileDocument apps on iOS18 simulators.