Turns out I’m not alone - I managed to find this on Reddit, with someone having a bunch of these pink bordered cards, including my Flaeron - note they’re also all from Jungle!
reddit.com/r/pokemoncardcollectors/comments/jezedc/pink_bordered_cards/
Seems like people either don’t know or assume sun damage - in a previous attempt of me finding out (a whole 5 years ago), someone else believed this was something they’d come across at some point:
reddit.com/r/pokemon/comments/5cexa5/_/d9w7oyz/?context=1
… don’t read too much into the comments, I was a kid 
The only other possible connection, although the cards look quite different, is this thread about sun damaged cards: www.elitefourum.com/t/is-this-a-printing-error/20705/1
Edit: … seems the reddit post / image may contain sun damaged cards, note the Pidgeot’s south border.
Perhaps it’s sun damage after all, since the color effects seem to be the exact same, so it likely was the same process to get there :’)