Strange bug happening since a couple of weeks.
The rectangle material is simple full white with shadow
This is happening on Chrome, Edge and Firefox (latest versions) on many elements of many artboards at 100% zoom and some time at 50% too
Sorry to hear about the trouble, we'll try our best to help you with it. Will it be possible for you to share the version of XD and the Operating system along with the Prototype link so that we can test this on our end?
I am having the same issue and don't know how to fix it.
I am sorry to hear you ran into trouble with this issue for a long time. I will try my best to investigate the issue. A few details would be helpful-
Please share the above details and will escalate it further with the team.