Cracks in the interior plaster / window reveal

Well, we have a thermally induced component contraction.

I do not disagree with that either. But the damage pattern does not look like normal movement. From a distance, I would rather say that these are poorly fastened windows where the frame clearly resonates when the window is slammed shut. Especially in picture 1.

If a certain movement in the window would be normal (which I cannot judge), the cracks would indicate the use of a rigid material in a place where a more flexible material would be appropriate.

Thank you for the answers, which fit the installation situation very well. The apartment is on the 4th floor, southwest side, meaning the sun shines fully on the windows in summer. Additionally, this is the weather-exposed side, meaning the wind load is occasionally high there.

I think when the apartment is being painted, the painter can fix the cracks and create a seal between the plaster and the window with silicone. Until then, I will check whether the fastening screws of the frame are tight on the sides. All in all, the effort should be manageable.

Thank you very much, you have helped me a lot!