I have the same issue I just realized that it hasn’t been backing up my video since a while. The last auto-uploaded video is dated November 1st 2017.
Running Nextcloud Android 3.0.0 RC1 and Nextcloud 12.0.4
It is not the first time autoupload has such a major issue. Can’t there be something that can be done to make it reliable (unit testing ?) ? It is certainly the most important feature of the app: keeping secured private souvenirs.