Not entirely sure how this is possible as I wasn't aware it was saved as a tag, but I've now double checked this using two different audio files and it's definitely happening.

When saving a 5.1 audio file in Picard (with Rename, Move, Save Tags enabled) it seems to change 5.1 audio files to 5.1(side) mapping instead.

The target files in question are 6 channel FLAC files. Here's one of the files being played in MPV before being tagged with Picard:

{{ Audio --aid=1 (flac 6ch 44100Hz)
File tags:
Artist: REM
Album: Automatic For The People
Title: Drive
Track: 1
AO: [pulse] 44100Hz 5.1 6ch s32
A: 00:04:30 / 00:04:31 (99%) Cache: 0s}}

Then, after tagging with Picard:

{{ Audio --aid=1 (flac 6ch 44100Hz)
File tags:
Artist: R.E.M.
Album: Automatic for the People
Album_Artist: R.E.M.
Comment: Chillout (Songs-DB_Occasion);Male Vocalist (Songs-DB_Custom2);Singer-Songwriter (Songs-DB_Custom2);American (Songs-DB_Custom4);Athens (Songs-DB_Custom3);80s (Songs-DB_Custom1)
Date: 2002-12-03
Genre: Alternative Rock;Alternative;Indie Rock;College Rock
Title: Drive (5.1 mix)
Track: 13
AO: [pulse] 44100Hz 5.1(side) 6ch s32
A: 00:00:10 / 00:04:31 (4%) Cache: 259s+118MB}}

Notice how it's changed from 44100Hz 5.1 6ch s32 to 44100Hz 5.1(side) 6ch s32.

Now idea how or why Picard is doing this, especially as to fix it requires reencoding it with FFMPEG to change the speaker mapping back.

(Style >> Preformatted doesn't work by the way, the curly braces don't seem to do anything)