Change of focus from RDP session inserts a character

This is a pretty obscure issue that started a couple of weeks ago, it is really interfering with my work, and so far I'm unable to come up with a solution. When I have an RDP session open, and am working on something text-based on the remote host, i.e. a powershell script, when I change focus from that session by clicking on something like a browser on my local desktop, a character is inserted in the RDP session, specifically, a "5". Every time I click on the RDP session, then away from it, another "5" is inserted. As you can imagine this causes havoc with the script I was editing on the RDP session. This happens when my session originates on either my Windows 10 or 11 desktops, and happens regardless of whether I use the standard mstsc client or another RDC manager. I've run a full AV scan on both hosts and nothing came up. I'm willing to format these systems and start clean if necessary, but I don't like the idea of not knowing the cause, so any thoughts on where to look or what to try next would be very much appreciated.

Thanks.