I don’t think the timezone set on Trakt can be related to the issue.
Because the timezone set on Trakt only “translates” airdates for episodes/calendar, not watchtimes on profiles (your own or other people’s).
So even if I change my timezone from GMT+1 to GMT-3, the latest movie watched will still be 12 May 2025 23:13.
The device timezone though, maybe that could screw things up when setting things as just finished/watched (as your computer would tell you it’s 21:19 when it’s for example actually 20:19 in your location, and Trakt would use the computer timezone time instead of actual time).
But with manually that still shouldn’t matter, it should use the time you entered, whether it’s the current time, an hour in the future, or in the past.
Weird, but interesting.