These links are not intended as possible fixes; but, to indicate similar issues which may have the same root cause;
- it says sddm-greeter-qt6 doent mix qt version (6.9.0 and 6.9.1)
- sddm-greeter-qt6 Failed to build graphics pipeline state
- sddm-greeter-qt[748]: segfault
- sddm-greeter-qt6 Failed to link shader program
It’s an annoyance that has been lingering for quite some time. 
It seems to happen with a fairly limited amount of systems, but that’s a long way from being definitive.