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 similar issue (much milder though) happen after I wrongly set the internal bridge flow to the same value as the external bridge flow. This caused extra thick internal bridges ruining the layers on top with a bumpy texture.