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.
How old are your printer, filament and slicer? Zits are common in adolescents.
And now serously, if you look closely at how inconsistent your lines are, I would definitely look into the extruder. Maybe you have power issues, that could account for both printers having the same problem. If you suspect it’s the slicer, simply install another one and try.