"Kill anything that moves" unlocked early for other player in local co-op

[PraiseTheFluppi]

I'm currently trying to get all the co-op trophies in this game in order to focus on the single player ones. I do this in local co-op mode with my brother's account and a second controller, because the servers are... pretty dead 😅 And local is easier to coordinate anyway!

Just now I got the trophy "Kill anything that moves" (kill 5000 enemies of all types), which should be pretty accurate. But what confuses me is that my brother now also got the trophy, roughly 10 zombie kills later, even though his game profile shouldn't have more than 200 kills. I only took his character with me on 7 quests so far.

I've read that there are a few bugged trophies (in positive ways), but I've not seen this trophy mentioned yet. Has anyone else experienced this before? This could potentially be exploited, even though I unfortunately don't know how this trophy got triggered.