I’m a broken record on moisture in filament but whenever something used to print properly and now doesn’t, it frequently can be water. With printing in PETG, it’s even more likely since it’s a water magnet it seems.
Have you dried your filament? How do you dry your filament? How do you store it after drying?
Not all symptoms are exactly the same since much depends on the model, ambient temperature and humidity, etc.
Moisture may not be the issue but trying to parse what you said in the OP, it sounds like it might be an issue especially since it sounds like you’ve eliminated a lot of other possible causes.
Spoiler - this person’s issue looks like it was water.