6 months is over kill.
I used to spray new builds and you could go on with a mist coat whilst the plaster was still wet. A spray mix coat is 50/50 of 60/40 water.
This usually happens when someone doesn’t apply a contract matt paint, as the mist coat.
If your mist coat has vinyl in it, it will still not adhere well, no matter how much you dilute it.
As a quick fix I would sand around the immediate area; looks like heat and moisture above the shower, is getting between the plaster and base coat.
You could even scrape around area to see how far you get till the paint seems more stable.
Then go over what you’ve exposed with a watered down contract matt. Put the first coat on thin 50/50. let it dry properly; min 4 hours. Then apply another slightly thicker coat say 75/25 contract.
Let this dry properly.
Then go over with vinyl emulsion. You can use eggshell if you want.
If you continue to get peeling, keep chasing it in the same way. If you’re lucky, you should only have to do the immediate area.
Once you’ve solved the problem, then go over the whole ceiling with a fresh top coat.
You can then watch in horror as your roller sucks of random patches all over the ceiling.
Unfortunately misapplied paint is a real nightmare. I had an entire house that way. I ended up stripping the whole house. The paint came off like wall paper.
I generally avoid things like Zinser. Their stain blocker is good but if they could sell you a paint called ‘house builder’, they would.
