I know exactly why as this same thing is happening to me and a few friends who have very high rounds. It has to do with your time survived on the last round.
I believe the glitch started at around 18 minutes of round time.
So it will depend, how many players are your playing with? If its 3-4 then your round are very long past the 20's and you could be dying past this 18min mark.
Right now I am stuck on a 48. I have been to round 53 the time that I died about 20-30 minutes into the round.
This is the bad part. Hopefully you remember the first game that didn't count. The VERY HIGHEST round that you have been to is the round that you now have to beat.
The game somehow still knows that I've been to round 53 @ 20 something minutes in.
I know this because I have gotten a 50 that didn't register as well, also made sure I died right at the start of the round to make sure.
So I need a round 54 now for the leaderboards to UNGLITCH my highest round.
This is was you need to do also, beat the highest round you've ever been to by a few rounds, then die at start of the round.
Also, I believe in the most recent patch this last week this glitch has been fixed, quite a few top players now have rounds recorded over 18 min. Before they did not.
Hope this helps....
Oh ya whats your highest round on Nacht? I love to play that map but no one plays. Right now I'm a 39 which I know not many have been to legit. Been trying to get a 40 for a while now