The kernels (and accompanying modules/drivers) are more or less freely interchangeable.
Bugs in the kernel are pretty rare in my experience. I think it’s more likely that the bug was somewhere in KDE Plasma. Kubuntu’s version should be older than the one on Cachy. On top of that Kubuntu has their own patches for KDE, so even if the version numbers are the same they are not the exact same programs. And on top of that the way they compiled KDE will be slightly different.











Newest version is 6.6.something so maybe the bug is fixed by now.