Is the knee wall too low? What does the measurement refer to?

This strange strangely dashed line that runs diagonally and goes over the door, no idea what that is supposed to be.

The dotted line probably marks the height of 230 cm, the dashed one is at about 240 cm and could indicate insulation below the rafters (?) In an attic floor plan in a thread where you asked for ideas about bathroom planning, a 1m height line is also drawn in, which would make little sense at a 150 cm knee wall. Has the attic been redesigned since then (even though the eaves and ridge height apparently kept the "former" value)? Somehow/somewhere there always seem to be measurement misunderstandings, I vaguely remember first windows and then interior walls suspected of being positioned incorrectly by 10 cm.

And why extensions from the window? Do floor-to-ceiling windows

I was thinking of roof windows (which logically would only be above the bend).

In a loft floor plan in a thread where you asked for ideas on bathroom planning, a 1m height line is also drawn, which would make little sense at a 150 cm knee wall.

You surely mean the 2m line, I cannot find a 1m line. Nothing has been changed since then.

Somehow/somewhere there always seem to be measurement misunderstandings, I vaguely suspect first the windows and then the interior walls to be positioned wrongly by 10 cm.

The issue with the window was probably my fault, although I measured five times during the shell construction, an error slipped in somewhere (or was it an old plan I checked?) And the wall, yes, it was wrong and they corrected it immediately. However, the day before yesterday we also had our wastewater connection installed, the pipe was actually supposed to protrude about 50 cm out of the house, and the workers looked and dug and dug and dug stupidly. In the end, the pipe was exactly at the house wall, if not even a bit behind it. So much for working according to plan.

I was thinking of roof windows (which logically would only be above the bend).

Oh, I hadn’t thought of that.

The initial situation is, in my opinion, quite clear. The knee wall is 125 cm high (as also marked in the drawing). If the wall was built up to 125 cm high, everything was executed properly. The 150 cm measurement is also not wrong, but actually totally unnecessary and, in my opinion, somewhat misleading. The construction manager's email already states that, albeit a bit unfriendly. The rafters are not the same as a finished ceiling. Therefore, just like with the floor, height is lost there due to the construction. I personally find 10 cm quite a lot, but without details about the structural design, it cannot be assessed further. The matter is simple: If the built (or however constructed) knee wall is 125 cm high and the roof angle also fits, then the construction project has been carried out as per the plan and is therefore correct. Of course, the interesting question is how the agreement was at the time of contract creation and what was actually agreed upon in the contract. However, if the drawing was already part of the contract, then there is nothing to change about it. Personally, I find the whole thing very unfortunate because you expected something different and are now not getting it. A good contractor should actually go into so much detail with the customers to clear up such uncertainties; this does not seem to have happened here. However, my experience is also that many building owners often do not want to be bothered with the very technical details or simply lack the necessary understanding. Much more attention is focused on the interior fittings (floor coverings, bathroom sampling, etc.) and the actual shell is often neglected. Therefore, I also find it difficult to always blame the bad contractors if you yourself were not involved.

that many builders often do not want to be bothered with the very technical details or simply lack the necessary understanding.

In particular, laypeople often confuse (half) centimeter-accurate measurements with manufacturing precision, and after accounting for tolerances (and plaster / cladding, etc.), the favorite cabinet then regularly no longer fits into the niche where they had already placed it in their dreams.

In particular, laymen often confuse (half)centimeter-accurate measurements with manufacturing precision, and considering the tolerances (and plastering / paneling etc.), the favorite cabinet regularly no longer fits into the niche where it was already placed in their dreams.

That's true, I work in industry where I deal with mm and µm every day, but I originally come from construction where the phrase "1 cm is not a measurement" still holds. I always find it interesting how everyone wants "their individual house" to forcibly set themselves apart from the "standard." Standards often exist because they are proven and make sense. This is not about making a room bigger or smaller, but even the most individual car is entered through the door and not through the trunk, and the steering wheel is also accessible from the driver’s seat.
Likewise, I repeatedly experience among acquaintances that the house is planned around the current furniture. Sure, you don’t want to buy everything new, but most somehow forget that they bought their now so beloved furniture to fit their (rental) apartment. Friends of mine built a 45° corner into their kitchen (exterior wall) to be able to reuse the existing fitted kitchen exactly as it was. Can you believe that???
But enough OT: such things must be clarified with the contractor. Best also on site and in person, not by email. You can document everything afterwards, but in case of questions, dialogue helps. You often quickly find a solution this way, and the contractor is also more willing to make the customer happy than if he is politely asked to do so by a lawyer’s letter. You still have that option afterwards.