When logging in with X, I get the password dialogue for my ssh keys popping up before the desktop starts to render.
Under Wayland, the same thing is happening behind the scenes, but the screen is black and so nothing is displayed. Having realised eventually why the login hangs, and putting the password into blackness, the login then continues as normal.
This seems to be independent of the Plasma version as its been like this for years, and still occurs on KDE Neon Daily as well.