Issue with Indesign and random shaded shadows on the project

An example attached the random grey & black shading apears when moving around the project. It happens randomly within indesign.

I had an old 2012 iMac which was running a little slow and didnt support metal..

This has been replaced with a 2020 iMac 5K with 32Gb RAM 8GB GPU, 3.8 i& 6 core.

Any idea what causes this..?