Yes, I think I know what's going on. I'm working on a fix for this, presently.Hey there! First off I just wanted to say that I absolutely love this mod, and it really makes life easier.
Unfortunately as soon as I went into hardmode I started encountering problems. I tested this multiple times between different worlds and found something interesting. For the sake of expediency I will use the phrase HM World 1, HM World 2, and Regular World
If I start out in HM World 1 with mirrors that are discovered and collected, I can leave and load up Regular World, then leave and return to HM World 1 without losing any mirror data.
If I start out in HM World 1 and go to HM World 2, all mirrors will reset and when I enter HM World 2 any mirrors there will be undiscovered, and if I go back to HM World 1, all the mirrors there are undiscovered too.
Conclusion: Something about changing hardmode worlds is the root of the problem. I have no idea how to attach more data of what's happening but if you need I can try to look into it.
Thanks for your lovely mod and your time