I had the same issue today. Everytime when I tried to ADS it kept unscoping me. We did some trouble shooting today and found out that when you have the same button assigned to "Eye Focus" and "Toggle ADS" (in my case both where set to right mouse button), the issue occurs. If you reassign a different key to "Toggle ADS", the problem will be solved.
Down below a twitch link, to the solution that also shows the problem:
https://clips.twitch.tv/CuteVastMonkeyNerfRedBlaster-DKzvBPHSk0QApZbD