it's like they get dopamine hits flexing they read the manga
There's also the manga readers that get addicted to the dopamine hit from reading a good anime-only reaction comment and want more.
These addicts will try to increase their dopamine fix by making what they think is a "harmless" comment that will draw out further reaction.
Figured out years ago I had this tendency. I'd start typing a comment, pause, think "I'm leading on way too much here" and delete before posting. Now I strictly don't comment at all or only in the source material corner. Don't trust myself to comment without being blinded by a need to draw out further reactions so I can get a fix.
It was difficult to hold back in the latest CSM thread. Seeing anime-only comments talking about Makima's bountiful bosom or enchanting eyes while thinking "if only you knew what lies below that! Makima's booty is so bodacious that the deluge of fanart posts caused /r/manga to turn into /r/makima_ass for months!"