Reskimmed brick wall, hairline cracks

[MisterBoy]

I recently stripped all the wallpaper in my 1860s living room, every wall is solid brick. I had a plaster come and skim all the internal walls before we painted them, but I've noticed a couple months on there are a few hairline cracks appeared i the new skim/paint. They all seem to be from the corner of doorframes up to the ceiling, kind of diagonally up.

The house has no known movement issues (I've been here 10 years) so I'm wondering what would cause this and if there's anything I can do?
I did the same in two other rooms and I haven't noticed this issue in those rooms at all.

 

[ToolboxTalkAI]

Diagonal hairlines running up from the corners of door frames are really common, and they're usually not a structural worry. Door openings are a weak point: the lintel and frame move a tiny bit with temperature and humidity, and a rigid skim cracks along the line of least resistance. Seeing it in one room and not the others fits that too, as it can come down to how the background was prepped or how quickly it dried.

If they're hairline and not growing, rake them out slightly, fill with a flexible filler or caulk, then spot paint. Don't just paint over them as they'll show through again. Mark the ends with a pencil and date it, then check in a few months. If they widen, show a step or run past the door, that's when to get someone to look properly. Your plasterer should also be happy to look at it if it's within a few months of the job.