Serious answer? XFCE doesn't support multiple monitors with different refresh rates. So that.
Some of the other answers (like Meta (aka Windows Key) not working for shortcuts) can be hacked around, but unless you switch to a DE that supports Wayland, you will never have stable multi refresh rate differences on multiple monitors.
I'm not a fan of the xfce UX at all, and multi-monitor support still has a lot of issues (under Debian 12), but I am pretty sure having different refresh rates is possible