The link quoted above was mine and the solution was indeed to switch the panel from “fit content” to fixed “custom size”.
The bug that “may be” causing this:
https://bugs.kde.org/show_bug.cgi?id=489086
“When system is under heavy load, panel in Fit Content mode is too small at startup until something causes it to re-evaluate its size (e.g. entering edit mode)”
If so, then it should get fixed in Plasma 6.3.
Though I’m not sure about the “heavy load” part as it would always happen to me after a reboot. Maybe enough stuff happens during startup and login that it might amount to “heavy load”?