Got my failing R earpiece replaced at Apple Store tonight. My right APP just would not pass the fit test. It used to, but it quit some time ago. After reading this forum, I contacted Apple Support via Chat. After doing stuff I know would not make a difference, support created an in store appointment. Went in, and the right earpiece was replaced under warranty. I kept the same Large tip that came with my original AirPod Pro. With absolutely no fiddling, both sides pass the fit test. And I can tell the difference. Low sounds that once were canceled out are now gone again.
FWIW Firmware is 2C54. Specialist said both earpieces must have the same firmware to work.