3 virtual desktops on Windows Server, RDM only connects to one account regardless of credentials used.

Use Case: We have 3 virtual desktops (VDs) on our windows server; assigned to Sales, Purchasing and Accounting. In many cases we will have reason to access more than one desktop at a time on the same PC. We are evaluating RDM to save the time and effort spend typing and retyping of credentials each time we change from one user to the other. Using RDP I have no trouble opening each VD independently from the same PC, aside from the time and effort wasted retyping credentials each time.

Problem: I have successfully added the Accounting VD to RDM, but every other VD I try to add will resolve to Accounting no matter which credentials are being used. The credentials for Purchasing will open the Accounting desktop even though the username clearly calls out "servername\purchasing". I'm at a loss as to how this is happening, I should not be able to connect to the Accounting desktop with the Purchasing credentials.

Details:
All VDs use the same password (could this be the issue?)
All have identical host and domain settings with the exception of the username (servername\accounting, servername\sales, servername\purchasing)
All are hosted on the same Windows Server 2019 Std
We own 3 RDP licenses for concurrent use (which works fine when using independent RDP instances on the same PC)

If we can solve this issue, we will purchase the full version.

Thank you

Stephen