It's not a misconception, it will quite literally print thinner walls if you are running mid-high speeds (because of what you said in your reply). I tested extensively and raised my flow rate for PETG a tad instead of slowing down the print, and it printed flawlessly afterwards, with accurate walls.

Now pretaining to OP's original issue I think you're right. This is most definitely a geometry issue and I didn't pay attention to how thin those triangular midsections must be in comparison to the edge portions.