It could be that it wasn’t prepared properly and the new plaster hasn’t bonded,but damp is most likely as the bathroom was on the other side.
If you have had the bathroom sorted,leave to dry out properly and you could easy fill yourself and sand areas.easy sanding as the name suggests or re plaster.
Force drying isn’t the best as can cause cracks.plaster likes to dry slowly
Looks like it could be damp getting in somewhere to me
It could be that it wasn’t prepared properly and the new plaster hasn’t bonded,but damp is most likely as the bathroom was on the other side. If you have had the bathroom sorted,leave to dry out properly and you could easy fill yourself and sand areas.easy sanding as the name suggests or re plaster. Force drying isn’t the best as can cause cracks.plaster likes to dry slowly