I've done some testing at my test forum, and I think that I figured it out. There is something wrong with the default regexp setting. When you enable the extension for the first time, the hashtags do not render. But if you edit the regexp setting to anything, save it and then remove it so it reverts back to default, it works.
I assume that this happens because this extension doesn't define any migrations in it's source code. So, when you enable the extension for the first time, the regexp field is blank and nothing renders. When you edit that field and then reset it, only then is the default regex saved to the database.
So yes, saving the settings at least once and then reverting would work (as 010101 pointed out), but not everyone is aware of this. IMO this extension should define a migration
