If the face it occurs on does not change based on the orientation of the model on the build plate the problem is one of 2 things, Environmental or hardware. You have eliminated bad slices or not enough support for example by reorienting the model.
so let’s delve a little deeper into Environmental I believe you have touched on already but I will list it as a matter of troubleshooting and anyone else reading the thread. Usually environmental will include anything in the room but not directly on the printer, Heater ducts Windows, AC units, Open closing doors, High traffic area that people walk in front of the printer often, the cat may like to lie on a nice warm heated bed. Yes If you are still reading you may think I’m being ridiculous but it has actually happened.
So if there is nothing in the room right now that may be affecting it let’s look at the printer and what can be affecting this.
First off the question would be did this print in particular ever work properly? Have you tried the print with varying degrees of FAN speed? I generally print my PETG with 0% fan unless I have really small layers times as it really does not need it.
Have you tried the same Gcode file with a different filament? If you want to try PLA instead of PETG, Drop the temp and increase the fan after the print starts and see if you get any difference in quality.
If there is no difference up to this point I would look at a bad nozzle, Replace it and try, they are pretty cheap at this point and its looking like you are into a maintenance cycle at this point so lets get it out of the way. Check for broken fan ducts, Fans not spinning at the proper speed, Partial nozzle/throat clog, Bad /Loose Vwheel. Bad or loose extruder gear, Broken arm or spring on the extruder. Clogged teeth on the extruder.
Sorry I know that’s a lot to check but lets stop there for now, Just follow it in order you will find it pretty quickly I am sure.
I am getting the same issue with PLA and other files. Its only the one side of the print that is bad…the other side looks beautiful. Its a hardened steel nozzle and prints smaller flatter objects with no issues. the thing that stumps me is that its only one part of the print…I could understand it being any of those if it was the entire print but its a large object that is fine except for one side. I’ll check those, one area that was suggested was a loose belt so Ill try that too.