[sobellian]

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.

[m4rtink]

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.

[chasd00]

Iirc cold gas thrusters are used before ignition to provide some acceleration to force the fuel to the bottom of the tank.

[baq]

The technical term for that is 'ullage'.

See also: https://en.wikipedia.org/wiki/Ullage_motor