I am having a strange problem with a solution deployed to a SP 2013 on-premises server. The solution is using some third-party ASP.NET controls which were previously deployed in the GAC. For some unknown reason these third-party libraries are removed from the GAC. This happens once every couple of months (sometimes more often). We are suspecting some administrative task or another solution is deleting it, but according to the administrator they haven't done anything about updating SP or installing (or removing) a new solution/feature.
Since the controls does not need to be in the GAC, we moved them to WebApplication (the safe controls were added manually to the SP web.config). I hoped this would solve the problem, but after several months of working correctly, the users experience some crashes of the application. For some reason the ULS logs show that the libraries could not be found. As far as I understand these problems happen for a couple of hours, and afterwards it goes back to normal (without anyone doing anything to fix it).
Any ideas what might be causing this?
