No matter if i'm in standalone mode or in a DAW, then i have no such issue.
I do this:
1. Click the "Learn" button in OP-Xa MIDI config section you show.
2. Click the PWM button you mention in OP-Xa
3. Click a button using the midi CC#55 you mention. The midi CC is assigned. All excisting assigned midi CC's does'nt change at all.
When i then disable "Learn", then all works fine.
I started out with the "default" MIDI config, like it look like you did. (You can try start with an "Empty" to see if it make a difference for you.)
Do you have the issue, both in a DAW and when using OP-Xa in standalone mode? Can it be a DAW related issue?
Is it the same no matter which midi CC you use?
Is it happening in all your Arturia applications? (you can allways try to reinstall one to see of it make a difference.)
Which OS do you use?
FYI: You should be able to upload up to 5 attachments per posts. It's okay you have done like you did.