I don’t know if this is actually a bug or not, but it has already been asked once on the forum and there hasn’t been an official answer as far as I have seen. So…
The minis vanish all the time. If you use a waypoint (sometimes), change the map, go down in battle, deposit things in the bank (unless you have an invisible bag) – it’s gone. And you have to actively look for them to activate them (which I often forget since the invisible bag is way down in the inventory).
Since there are more and more minis that can be bought and received from achievements, I suspect that you WANT us to use and collect them. So why are you making it so hard?
Why do they have to vanish all the time? And why not have a separate mini slot in the inventory that automatically activates them? (Or any other improvement that makes them more useable.) I’d even pay a few gems for that.