"The emulsion in one room upstairs had become flaky/cracked....seems like newish plaster underneath."
Rub your hand over that exposed plaster and see if it seems to be particularily soft and/or if it rubs off the wall more easily than you'd expect. If so, then I think I can explain what's happening...
Emulsion paints will develop quite a bit of tension in the paint film as it dries, and if that paint film is sticking to a soft powdery surface, the tension in the paint can be strong enough to pull the soft plaster the paint is sticking too off the wall. In that case, what's actually happening is that the paint is sticking to the plaster well, but the plaster it's sticking to isn't sticking to the plaster behind it very well. So, when the emulsion paint dries and you get tension building up in that paint film, that tension can be enough to pull the surface layer of plaster off, and the result is that we see paint cracking and peeling, and we presume the problem is with the paint. It might not be the paint at all, but that the plaster on the wall isn't strong enough to resist the tension that develops in ANY emulsion paint as it dries.
(alternatively, maybe whoever paint the patch didn't wipe the sanding dust off the surface before painting)
Here, try this:
Paint over that "newish plaster underneath" with a high gloss colored emulsion paint, and wait for it to crack and peel. Then examine the back side of those paint peelings, and if they seem to have a white powdery film on the back of them, then that's the plaster that got pulled off the wall's surface by the paint. And, that means the paint didn't let go of the plaster on the surface, the plaster on the surface let go of the plaster behind it. In that case, you just need to scrape that soft plaster off and go over it with a compound with more glue in it so it's a tad harder to sand smooth, but is stronger and holds together better.
Another check to confirm is to paint over that newish plaster underneath with a colored oil based paint. Oil based paints DO NOT develope any tension in them as they dry, and so if the oil based paint doesn't crack and peel, but the emulsion does, then that confirms that it's the tension in the paint that's causing the problem, and if the paint is sticking well to the plaster, the root of the problem lies in the soft powdery plaster it's being applied over.
Or, at least, based on what you're saying, this is the potential problem I'd be focusing my attention on.
Post again if you want to know why emulsion paints develop tension in the paint film as they dry, but oil based paints don't. It's not an awful lot of tension, but it is enough to cause paint problems over a weak or deteriorated substrate.
