Residual moisture in walls after two months - explanation?

Hello,
maybe someone can find an explanation.

On a room wall, plaster including insulation (Styrofoam) had to be chipped off on an area of about 30 x 40 cm down to the base wall of the exterior wall. In total, about 4-5 cm were removed.
Subsequently, new insulation was applied to the wall and then the rest was closed with plaster.
A day later, it was wallpapered with normal textured wallpaper and painted with silicate paint.

The wall was dry beforehand. There are no pipes running here or any other sources of moisture.

Two months later, I happened to see that the wallpaper has a crack exactly within the outline of this area. Then I measured the moisture with a Trotec and was amazed.
Normal values of the wall are 20, in the new area up to 60-70 digits.
Values increase exactly where the new plaster is behind the wallpaper.

The executing specialist company was informed on site. No explanation. Except to break it open and redo it.

All my logic says, since no moisture can come from outside, it can only be due to the plaster itself?
But no metal or anything else was embedded that could falsify the values.

Surface temperature matches the rest of the wall (18-19 degrees), which probably also suggests that the new insulation fits.

Does anyone have an explanation why residual moisture can still be measured after 2 months? I also specifically heated the spot to improve the values. So far without success.

Pictures were taken directly on the day of the repair

Where is the water from the plaster supposed to go when you are going to wallpaper immediately? I let it dry first and then wallpapered.

I would have too.. but the painter did not. However, I do not know if the woodchip wallpaper allows no moisture through at all...

Depends on how it was wallpapered, or rather which wallpaper paste was used and how thickly it was applied.

No, Dr. Motte, that's not the case. The problem is more in the color. If you use something good, even something colorfast quality, then it is quite dense. Not 100% but still quite dense.

Then maybe we should first discuss: would drying out through the wallpapered side into the room be desirable here or not? And I stick to this: the best, diffusion-open paint is useless if the wallpaper has already been glued on with plenty of paste and all the pores are already sealed from behind.

Or do you really think: the moisture should take care of itself on how it will ever get out, we paint over it now 100% waterproof?