Great it worked for selimrecep and Erwin, but it didn't for me, I updated (against the Tablet's (Microsoft's) advice) to Intel graphics driver 27.20.100.9168. Is that the same one that fixed it for you two? I had previously updated driver/firmware on windows update to 17763_20.020.2886.0. My issue only happens with whiteboard and as others have pointed out, only when using solid not rainbow colours - was that the same for you?
I also have had screen brightness flitting whilst the auto-adjust brightness was set - so have had to disable that.
Actually I don't know Erwin but the issue was same for me, after upgrading drivers it was fixed. Can you use solid colours when ruler is open, though? I was able to do use them when ruler was open.
If helps:
Windows 10 20H2
19042.746 (I don't know what is this, I found this on Windows Update)
Intel Iris HD Graphics 630 with 27.20.100.9168 version (seems like same with you)
Gtx 1050 Ti with 461.40 version Driver