Cannot remove Track Number from FLAC

Hey guys, so I've been using the program to edit my FLAC collection and for the most part, it's fine, however, there are some files where I remove the track number, however, on other tagging programs the track number is still showing up. It also shows up on my phone when I use USB Audio Player. There are also some files where the track number does not show up on other tagging programs but it shows up on my phone anyway.

On MP3Tag itself, the "track number" entry is blank, but this is clearly not the case.

How do you know?
What about the databases / caches of the various players?
What happens if you delete the file from the player's database and re-import it?
Or use a renamed version of the file to see if the metadata is read correctly.