I think you must be misinterpreting what you are seeing in the webUI. Any drive that isn't mounted will appear as an Unassigned Device. This is true even if you don't have the Unassigned Devices plugin installed.
This is not at all the same thing as saying the Unassigned Devices plugin has mounted the drives. Probably the drives became temporarily disconnected for some reason and when they connected again they were Unassigned, since Unraid will not try to remount them.
Post a screenshot of what you are seeing, and also go to Tools - Diagnostics and attach the complete diagnostics zip file to your next post.
I think there is a good chance your problems have absolutely nothing at all to do with this plugin, so I may split your issue into its own topic after we take a look.
I see you already have a thread going about your cache dropping so most likely this is just another symptom of that. If so, I will split this into that thread.