I still have Leap 42.3 on one of my computers, though I rarely use it.
When I last used it (to update in early November), Plasma was malfunctioning. I logged out and logged in again, and all was fine.
My experience is that if I delay login after boot for long enough, then this doesn’t happen. If I login too quickly after boot, it does. I think there’s a timing issue when the system is still busy from booting up.
I have not had this happen on 15.0. But maybe something like that is happening for you on 15.0. If so, it is probably a Plasma bug to have anything where the timing is that critical.