Ok. I guess there should be a different repository in our cases. Here is why I get:
aario@openmandriva-x8664:~$ dnf search qt6|grep xml|grep devel
Failed to set locale, defaulting to C.UTF-8 <--- Yeah....
Last metadata expiration check: 12:16:23 ago on Thu Feb 6 09:59:34 2025.
lib64Qt6Scxml-devel.znver1 : Development files for the Qt 6 Scxml library
lib64Qt6ScxmlQml-devel.znver1 : Development files for the Qt 6 ScxmlQml library
It shows those of Scxml instead of xml.
I have these repositories enabled: