This was a side effect of the new system, still trying to point to what exact event triggers it. Would only be cosmetic though, as the list that uses the true juggernaut is refreshed upon the unfreeze, and selects players on blue (true being the stuff with stat effects) Committed some changes to the code which should correct this
Speaking of the new system, the spawn uber is also a bit weird. Today there were some engineers hiding in a spawn room on freight, and I immediately gained uber when I attacked them inside the spawn allowing me to kill both of them easily. Also, one time, a medic approached us with seemingly infinite uber and there was nothing we could do about him. The walk from spawn to our spot was at least 15-20 seconds and he somehow still was ubered, so he could just walk into our nest and wreak havoc. The uber did eventually run out.
If that respawn room was blues that is the desired effect. Unfortunately to remove it from non spawn-spawn rooms (maps with two rooms or more), I would need to grab the coordinates of the extras per each map. Don't have time for that currently (Valve do not call them bluespawn2 etc, so manually is the only way) The longest uber they get is 2.5 seconds (Upon leaving). Inside the room its applied every 0.1 seconds and does not stack. That is an odd one, you sure they didn't walk into a secondary spawn room before approaching?
Nope, this was a medic who had come from the gold rush first spawn and just walked into our nest with uber. We were inside that mine shaft tunnel to the right that loops around to the windows that lead to the second cap. It's been a little while since then but it was definitely after the spawn changes. It's happened a few times on turbine as well (medics walking out and coming into the blue vents with extended uber)
Alright, what I have done was removed the 2.5 seconds on leave uber. So now it only applies while they are inside the respawn room. But I have also made it print to red people who go into these spawn room areas 'Warning you are in a Zombie uber area" I need to wait until I can upload it, as Swinburne blocks access Edit: Uploaded