I’m having exactly the same problem after upgrading from 4.8 to 4.12. The exact method you describe was working perfectly in 4.8 but has since stopped working along the way. I’m going to diff the various files involved and see if I can find something that might have stopped this working.
Update: OK it seems that this changed at some point between 4.8 and 4.12 but now if the actor that owns the particle system component you are trying to re-activate is hidden then the particle system emitters will not be reinitialized.
One work around is to make sure you unhide the actor before you call activate() not after.