I’m printing some trays that are thin on the bottom maybe 2mm. Not enough for there to be infill. I’m having the same issue on two completely different printers with different filament and different files. I’m using the same Orca profile for both of them though so it has to be a setting, maybe bed temperature? The sides of the trays are perfect.
I had a somewhat similar issue on large layers because of insufficient temperature for the speed. Large layers allowed the toolhead to accelerate to higher speeds than normal, and print temp became insufficient for layer adhesion. On regular walls this wasn’t an issue because the printer couldn’t go that fast. Slowing down or increasing the temperature helped.