I'm having a similar issue, I'm using "Mouse without Borders" as my KVM (no I can't use multiplicity I tried, blocked by my work computer)
One more thing that I noticed, clicking on the start menu seems to send the cursor to the top-middle of the main monitor. It does not happen when I use the default windows 11 start menu (Start11 taskbar but default OS start menu) through disabling the start menu or ctrl+click the start menu button.
I have not tried the custom button workaround yet I will and report back.