That's what I was thinking too, so I cut it open.
This is the underside and backside of the left wall. It looks dirty but not really damaged.
I pulled back the vapour barrier and the wood looks fine. It feels normal when I stab it with a screwdriver too, it's not mushy or anything.
The nail holding that bit of drywall on was rusty though.
I'm leaning towards it being something that only happened once. The discolouration around the crack in the floor and how well it aligns with where the staining on the tack strip starts makes me think it came in through there. Which has me wondering if I should seal that crack with something or just leave it. This corner will be the finishing end of the flooring so it'd be easy to remove later if needed, just pop the trim off and unclick a few planks.