I *thought* my periodic gameplay effects (Gameplay Ability System) weren't being locally predicted on a multiplayer client (quick ticking was janky). I wasn't doing anything wrong, it's because #unrealengine specifically disallows local prediction of periodic gameplay effects. Not mentioned anywhere that I could find, but it's right there in the source code 😐
This makes periodic effects a bit useless in multiplayer, except for coarse-grained loosey goosey things. Time for another approach