I'm having the same issue with 13 Pro Max - loose at the bottom and constantly moving slightly. I thought I was get used to it but it just never stops.
Does anyone tried putting something to fill the loose space with something?
Putting a piece of white cardboard stock works fine and fills the space nicely. I bet if you were good with scissors, you could cut it in a rounded way like the magnet cutouts and it would look like part of the case.