• How’s the environment? I mean do you print in a heated enclosure or free air? Or something else?
• The bottom layers do they look like this from layer 1 or does this only occur in the top layer?

Whenever I see bubbles I immediately think moisture. Since the sides are ok, I don’t think it’s moisture in the filament, but what about the air? That’s why I asked about the environment. It could be moisture or contamination (dust) causing localized layer adhesion issues.

My environmental hypothesis can also explain the acceptable sides. Since the layers with only sides are faster to print, those layers don’t collect as much of whatever it is between layers.

If it is environmental I’d expect to see the issue with every layer. Perhaps only noticable while printing because the nozzle will iron out the bumps.

And just to rule out the moist filament, do you dry your filament before printing? I kinda guess you do, because you have multiple printers. But asking about drying is akin to IT support asking me if I have rebooted my computer.