Most Windows admins spend quite a bit of time in RDP windows. I can’t imagine how many RDP sessions I’ve used since I started in IT 15 years ago, but it’s conservatively more than 10,000 (4 times a day X 5 days per week X 49 weeks a year * 15 years).
In all of that time and all of those sessions, never once have I come across these artifacts in my RDP session:
It took me quite some time in Google to find the answer, and I ended up finding an obscure post mentioning something about bitmap caching. I disabled bitmap caching on my RDP session and the problem disappeared.
The feature prevents the RDP client from re-downloading the same bitmap file, saving bandwidth and improving the user experience. I don’t know what particular set of configurations caused this to happen, but now I know how to get around it.