Addons do not work randomly
I sometimes install other addons beside the defaults like uBlock.
I noticed that sometimes, it is recurring not one off, but when Tor Browser is restarted or 'New Identity' is clicked addons do not work. For example, keep an icon of the addon on the Menu or Bookmarks tab - to reproduce try restarting the browser or closing the browser and reopening or even try new identity button (doesn't always happen but you will run it to it sooner or later).
I know addons other than the default ones aren't supported but there may be another issue with Tor Browser as to why sometimes addons aren't working (haven't tested the default addons but they may also stop working) which should be looked into.
Using the 6.0.1 Browser on Linux 64 platform