I've got a couple of MP3s with an inappropriate "Grouping" tag. I cannot delete them permanently. Here is what I try:

• Load MP3 (album) and wait until everything is loaded and queried
• Put cursor on "Grouping" label in tags list
• Hit "Del" key
• Tag is deleted (original value becomes red, new value becomes empty)
• Save (tried both just track or entire album, doesn't matter)
• Grouping tag disappears entirely, which is expected
• Remove album from track list, or close/re-open Picard (doesn't matter)
• Load MP3 (album) and wait until everything is loaded and queried

Observation: the "Grouping" tag has appeared again

Expectation: the "Grouping" tag should be gone forever

Note that at the same time I can add tags, or edit (other) tags, or even delete (other) tags and that works fine in general.

This is a bug that is long standing since many years, so it's not a 3.x regression.