There were issues about this. The splash screen implementation doesnt make sense, as elements dont start loading while its displaying. It would actually make a lot of sense if apps could load in the background, and this was just a fullscreen overlay. But currently it just slows down the start and users have lots of time seeing that they use KDE. Also when switching the plasma theme, it always reenables, which sucks too.
Basically a request to work around Bug 357170; marking as a duplicate of that. > Also when switching the plasma theme, it always reenables, which sucks too. That's a separate thing. You can submit a new bug report to plasmashell | global themes requesting the the Breeze global themes not specify a particular splash screen. *** This bug has been marked as a duplicate of bug 357170 ***
I think global themes should include a splash screen, the problem simply is that the current implementation doesnt work. To have global themes work on ostree systems, they should exclude SDDM themes (or the SDDM themes would need fixing). But this again would only bypass an upstream/detail problem, so I will not request both of these to be excluded.