Snipe and his bud are on the right track , but I’ll add a few more comments and questions.
How much use did the bath get used before this process started? You said it was “well vented” but do you know what that really means? How big is the room, how many cfm’s does the fan move, what kind of ducting does it have and is it on a timer or humidity sensor? Most people (even pros) woefully undersized their bath fans, put them in the wrong locations and don’t let them run long enough.
sorry for the snarky homily.
LOML washed the walls - did she remove the cleaning residue completely with water before painting ?
How long were the walls allowed to dry before priming ?
If a heavily used bath and good clean + rinse , then the walls were really wet. Even if they seemed “dry” to the touch.
Which BIN did you use? There’s a “synthetic “ shellac that cleans up with water . If you used that - there’s your problem.
Fake BiN Shellac.
https://www.amazon.com/Rust-Oleum-Corpor...76L4&psc=1
How thick/heavy was the primer put on? How long was it allowed to dry before the topcoat ?
The alligatoring shown is usually a result of a still wet undercoat (medium) encapsulated by a quick drying topcoat.
I suspect that your walls were too wet and or contaminated like snipe suggests. If you used fake BiN, that made things worse.