In my experience, it doesn't even have to be right before it ends. Seven continues ulting without the cloud regardless of when you upgrade the duration.
When hitting enemies, for some reason, the indicator that tells you how much damage you did doesn't properly reset. Here is a clip of it happening with gun damage during lane. https://outplayed.tv/deadlock/zrbg0l
Here is a screenshot from that clip:
I will note that I have had it happen...
It can be seen at about the 28 minute mark, the creeps that are meant to go up York get stuck and never leave base. Here is a clip showing them stuck going back and forth, from OutPlayed https://outplayed.tv/deadlock/GMNw6X
As well as a screenshot:
Match ID: 39254851
Bumping this because I've encountered the same bug. Happened from the end of my previous game. Someone died before the kill feed message could disappear.