Thank you very much for the information, this is very helpful to me!
We had another similar report in our system. Thankfully, we already had an explanation for this behavior:
In Archicad 23/24 shadows were not rendered completely due to OpenGL 3.3 limitations. That is, after a certain amount of memory consumption, the program simply ignored shadow rendering. This might create a situation when you have a complex object and shadow fails to display correctly in 3D.
Since Archicad 25 however, shadows are displayed completely (as in for every object), which is similar to how it was in Archicad 20 or previous. The downside is, it will slow down rendering quite a lot, but you will have a correct shadow presentation in 3D environment.
Thus, we suggest turning off shadow in 3D if the computer has limited resources or if it causes slowness with your 3D navigation. I hope that it explains the situation here, and thank you very much for your understanding!
Technical Support Engineer