Dan S. MacAbre presented the following explanation :
In case no-one else has any suggestions, I find that those Benzyl Alcohol
paint strippers soften up things like that - even when you might not want
them to :-) I've used them on acrylic sealers when nothing else seems to
work, but it also seems to soften up some of those ready-mixed plaster repair
fillers, which can be annoying; but at least I know to be careful with it
now. TBH, it's the only sort of stripper I'll buy from now on, since it's
the only thing I ever found that works on emulsion, too. Takes hours to
work, though. And it's supposed to be non-toxic (at least it is ATM) and it
is low-vapour. Costs about £30 for a big plastic bottle, so might not be
economic for a small job, but if you will be stripping paint in the future...

I don't want the underlying paint surface to be damaged..