There is a glitch where on activating the uber charge, the medic that is the source of the uber isnt immune to damage. Experienced on class wars, the glitch is well known. I am not sure if this is a skivers or a valve issue.
If this is the bug I think you're referring to. It's a game bug not a server bug. It's been around forever.
If you uber, while you are under uber. It removes it. Since everyone is medic and trying to uber chain you must wait in till the uber finishes before you unleash yours. If you do, it removes your existing uber and keeps you un-ubered while the other medic is ubered. Hence when uber chaining you time it right with your ubers. Not activating it before the existing uber finishes.
That's also why you see it happen on our servers a lot more cause again, everyone is medic. People try to uber chain, and in regular servers not everyone is a medic and it's mostly just one or maybe two people at most.
(This part in the video shows it perfectly, he ubered the other medic while under uber, and thus when the other medic finished his uber it ended his uber)