Thanks for the input everyone.
Almost all my timepieces are imported and I've never had an issue until now, hence me disqualifying some of the 'usual' possible causes and thinking that I just got an in-spec but unimpressive movement in my watch.
I can't 100% conclude whether the movement has settled in or it was magnetised, but I can report that I am very satisfied with the performance now. I followed the Hodinkee demag method, and assessed the performance every (+/-) 12 hours for several days now and the watch seems to be staying within 3 seconds.