This is actually a contribution to this question: Remote Desktop intermittently freezing but I can't post on that one due to having just registered
Host: Windows 10 version 2004 (insider preview) Client: MacOS 10.14.6, Microsoft RDP Client 10.3.9 Devices are on same physical LAN, different VLANs
Anyway, I am not able to apply the suggested fixes because my client is a Mac. For me at least it occurs whenever a dialog box box pops up. I think it somehow has to do with focus being shifted without user input. I have no idea what significance that might have vis-a-vis RDP, but I am able replicate it pretty reliably