I wasn't able to repro it with a 2 mb attach, which isn't all that surprising for a short, quickie test (although it took forever for it to arrive and both accounts are in the same office 365 tenant.) I'll need to let it run a bit longer and see if i can repro.
It was actually interesting to watch the folder - the message arrived and several seconds (probably 2 or 3 but it seemed longer) passed before the category was applied. Not sure if it was due to the message size or was due to other issues (that machine seems unusually slow today).
The rule has no conditions, only the run a script action? (so it applies to all mail)