It sounds like a signal/connection negotiation delay for the monitor that’s having that issue and this is something that can happen with both HDMI and Display Port. You can do /usr/lib/plasma-dbus-run-session-if-needed /usr/bin/startplasma-wayland or if you’re using X11 do /usr/lib/plasma-dbus-run-session-if-needed /usr/bin/startplasma-x11 by switching to a TTY from the login screen and starting KDE that way this will give you a verbose output of it loading where you can possibly see where that monitor is having trouble. You’ll want to look for warnings or errors related to drm or Mode Setting also EDID and HDMI-A-1 or 2 which would be DP-1 or 2 for Display Port. Anyways check the verbose output of KDE loading for errors related to video stuff if any. If there aren’t any errors being reported, I get its annoying, but personally I wouldn’t worry about it.
EDIT: CTRL+ALT+F2 or 3 to get to TTY and CTRL+ALT+F1 to get back to desktop screen, the verbose output will stay in the TTY where you started the desktop from.