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
Serious answer? XFCE doesn’t support multiple monitors with different refresh rates. So that.
That's more of a limiation because of X11. KDE and Gnome do not support different refreshrates on multiple monitors as far as I know. Its the main reason why I never used multiple monitors. But on Wayland, this issue is solved. So if XFCE is ported to Wayland, they should also get this support for free I guess.