I've applied the Case Conversion to UPPER for _TAG for the test files and used the "Rescan ID3 Tags" option from Serato. I've also accidentally messed up my whole local library by first pressing the "Analyze Files" option which made Serato write all kinds of tags into my files.
I could not reproduce the issue where the color codes and locked beat grids have been removed by Mp3tag for Mac for the MP3 files. They still show as locked and the color is also present.
I could reproduce the issue with MP4 and Serato which shows as "corrupted file" after applying the case conversion to UPPER for all tag fields via _TAG. I've analyzed the file and it was in no way corrupted — so I looked further.
Serato is storing all kinds of internal metadata in the file's tags. As you can see from the screenshot, there is a field for BEATGRID and also fields for MARKERS, MARKERSV2 and so on. The contents of those fields consist of both upper- and lowercase letters and numbers, basically binary data encoded as strings.
So if you apply an all fields upper-case action, those Serato metadata fields also get converted to uppercase, which is confusing to Serato and breaks the functionality of those fields.
I might be able to hide those Serato fields with a future update of Mp3tag for Mac. For now, I recommend being more selective in the fields you apply the actions to.