I think this is mostly an xorg-server bug, but I imagine Kwin will need to be made aware of the new modifies stuff that's recently been added
Adding xorg-server bug details
*modifiers
KWin is feature frozen for X11. It won't be adjusted to any changes in X
I think modifiers are a wayland thing too Hopefully they sort out the xserver bug, any idea from your end why it's not working?
According to the information you provided on the xorg-devel mailing list this specific problem went away when you removed an old config key enforcing the EGL on X backend, correct?