Hello,
Thank you for reporting this issue.
We are currently investigating it, and we found the following thread, which appears to closely match the behavior you're experiencing: https://github.com/FreeRDP/FreeRDP/issues/3863
According to that discussion, enabling SoftwareGDI appears to resolve the issue. At the moment, however, RDM does not expose an option to enable this setting.
We'll be running some tests today to determine whether we can add support for this option in RDM.
In the meantime, you could also try disabling "Persistent Bitmap Caching", as it may help. You can find this option in the RDP entry settings. Click "Show all properties", then go to the "Experience" tab and uncheck "Persistent Bitmap Caching".
Best Regards,
Frederick Simard