Hi @Stein99,
Thanks again for the detailed update. Since I haven't been able to reproduce the issue on my end with the same InDesign version and setup, it would be great if you could share a sample file where you're seeing this behavior. This will help us test with your content and get further analyzed.
Also, could you please try connecting the external display to your Mac Studio using HDMI connection instead of USB-C(if possible) and let me know if the issue still persists? There could be a rendering or scaling difference affecting the UI display.
Looking forward to your file and your observations.
Abhishek