Probably mostly on high ping/High pop servers, if you try to go ADS while from a crouch position try to stand up, you can get stuck.
The video should show better than (my very bad English) words.
It seems related with the crouch to stand action somehow.
In fact, crouch to stand action have often a strange bounce https://youtu.be/F454TBiH2wY
This bounce often, but not necessarily, appear when the ADS problem is about to happen.
Mostly on high populated/high ping server, but not exclusively.
-Crouch and start to proceed fast pacing
-stop to proceed fast pacing the moment you push the button to stand
-few moments later try to ADS, you should be not able to or with a huge delay of a few seconds.
The stuck can consist in you unable to ADS for some seconds or sometimes you have to push the ADS button repeatedly since you come out from the stuck situation.
I have this problem on an high populated/high ping server (Those video are mostly from DayOne Livonia server) but I have been able to reproduce the problem even on Vanilla official server.
The difference is the reproducibility.
On DayOne (high pop/high ping) I have a reproducibility of at least 50% while on Vanilla I spent couple of hour to have the problem happen only few times.
It can have maybe something to do with loading ''new assent''.
I say so because the only times I could be able to reproduce it on Vanilla server, something new was happening on my screen (passing next to a chicken, passing from an empty zone to a zone packed with zombie, fights scenarios).
So in the equation seems to be involved server load somehow, even without high ping/high population.
The high ping/high population on a server skyrock the problem to unplayability.
https://youtu.be/6URnl7TBazg
https://youtu.be/szJdo6azaDA
https://youtu.be/p85VjgnagAw
https://youtu.be/BsK94l_UAcc
My system is:
i7 6700k @4.5 Ghz
Nvidia 1080Ti
256Gb SSD Samsung 850 EVO M.2
16 Gb Ram
This is easier to reproduce on servers ran on AMD processors. This is plaguing the dayone server community as well.
I think I can say the problem have its origin in the CHANGE OF STANCE.
For some reason, on certain servers more than others, switching from a stance to another (mostly from crouch to stand and viceversa) have some kind of glitch.
The character seems to have a sort of "bounce" in the process.
It can be noticed even on the little icon on the lower left side. This bounce is the origin of the ADS issue, because it's the origin of the character stuck in general.
On the Official server I have a lot of difficulty to reproduce it, while on some private servers the issue is substantially constant.
On the Official I noticed that I can reproduce it more consistently if I spam change of stance during the exhaustion phase, where my character lose all the stamina and the regain start.