Cracks in cosmetic stud wall.

[Brienne]

As part of an extension, we had an archway between the living room and dining room walled off with a stud wall.
When the plaster dried there was a vertical crack and the builders have subsequently filled and reskimmed (x2) and most recently they have been back and ripped out the plasterboard and placed it running horizontally and I have never seen so many screws.
It's now been re skimmed and there is now a vertical stepped crack in it.
The original wood timber work was done back in Feb so I don't think it's the timber drying out with the heating.
The builders have been great and have come back repeatedly to try and sort it. I was wondering whether there was a bit of movement in the floor underneath but the most recent reboarding deliberately left a gap behind the skirting between the boards and the floor and the boards were properly bonded and that webbing tape applied...

We're all at a loss to explain it.
There is a tiny unnoticeable vertical crack on the other (extension) side of the wall.

We have no other cracks and this is def not structural. Just annoying as I want a decent cosmetic result.

Has anyone got any suggestions (beyond lining paper......)
I'll attach a photo of the original crack and one of the crack now.....

Bump

Could just be settlement/ effect of drying. I would leave just paint as you want it then see how it looks in 6-12 months and fill with polyfilla.

It could simply be that the original stud wall to either side of the closed opening was wet and is drying out beyond the filled -in hole. Because the crack is vertical its not structural movement related its most likely to be drying out as suggested. What is directly over the crack in the room upstairs? Is this causing an intermittent low loading leading to slight cracking. It could be a relatively weak floor joist beneath or that the stud isnt positioned directly over the floor joist. Concrete shouldnt budge.
It could also be that the crack is forming where the hole was infilled with a vertical stud with no lateral horizontal correcting studs and a limited number of screws. It would then seem logical that a new plasterboard was 'butted-up' to the existing, allowing movement under normal loading above.
I'd bet its shrinkage in the whole stud wall - answer, let it dry for a number of winter months, excavate the crack and fill with FLEXIBLE polyfilla, and redecorate - stick to emulsion until it stops cracking.
P.s. Plasterboards should interleave one with another for stability.