Heat pumps are very large internal volume AC system with a lot more extra piping, and a accumulator. The POE oil is very hydroscopic and breaks down, and turns into an acid when exposed to moisture with refrigerant.
Yes, I know why I feel slow flammability. It’s actually a little lower flammability by quite a bit then your grandmothers aqua net hairspray. I was one of those kids who went around with the lighter in my grandmothers hairspray as a flamethrower to burn spiderwebs in the backyard.. And I can tell you from first-hand experience why if refrigerant is less flammable than that