I saw one that color caught last week on another site.
Posted reason by local "expert fish IDer" was the fish was caught from dark water or possibly from under a rock.
Poster, said fish hit just as jig floated under a big chunk of rock. Gill came out from under and took jig back under rock.
Hope this helps. I do know the river the guy caught his gill from and there is no tannic acid in that area.
I'd say it was because of the fish being under cover.