At least for the retro-propulsive landing burn, I think the modeling problem is probably aided by the high G-forces that must keep the fuel very close to the bottom of the tank. Even before re-light the booster is falling near terminal velocity (I think?), so the fuel is likely sitting at the bottom.
I think it's a huge problem when re-lighting the engines in orbit, though.
Also IIRC the massive main tanks in Super Heavy should be basically empty at landing & the landings propelants come from a set of small header tabks that are near the central axis of the vehicle & arr completely full. This should reduce or even fully remove sloshing issues at landing time.
I think it's a huge problem when re-lighting the engines in orbit, though.