Display resolution changing

I'm having a strange problem with my display resolution. This doesn't happen with Gnome, so I assume it's KDE-related in some way. Briefly, I have an HP Pavilion 23xi monitor connected via HDMI to an HDMI switch, which in turns connect to two GPUs. One of these is an Intel 915 on the motherboard and is what I normally use for Linux. The other is an Nvidia GTX1050 which I use only for Windows gaming using a KVM/QEMU VM with VFIO passthrough. What this means is that the Nvidia card is masked (blacklisted) from Linux and is only visible to the VM. In fact neither of the Nvidia nor Nouveau kernel modules are even loaded. I now find that when I switch the monitor inputs from the Nvidia to the i915, my display resolution is being reset to 1280x720 instead of its normal 1920x1080. This happens *even when I'm not running the VM*. I have to manually reconfigure the resolution using the KDE Settings widget. This never used to happen, and I can see no reason for it to be happening now, but it has started since I updated to Plasma 5.20. Even if I'm running the Windows VM, the display is always set to the same 1920x1080, so I don't know where the lower resolution is even coming from. Any thoughts on this would be welcome. poc