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.
Could it be warping related? As in the ones that usually happens by lifting corners.
Another candidate or a partial subject is an uneven print bed. It does happend over time. If your printer has a layer test print you could check if the print result is even.
Anyway, both of the above can be alleviated with a slower print speed to give the extra time for print bed adhesion. Not really a guaranteed fix, but the results might be good enough.