Afaik, this bug trigger after you create a 9th slot, it never affected me before ive created a revenant (9th class/slot…) wich took me a good 2 months after release to get on my second account, my main account been affected by this bug since wardrobe release (yes… about 3 years ago…).
I cant imagine they are not aware of this issue, its mentionned since f.o.r.e.v.e.r. on the wiki;
Bug: Miniatures are not saved properly for all accounts, and sometimes unselected after a loading screen.
While this note did change over the years, and never mention the bug appears only after the 9th slot, it seems devs never cared enought to fix it properly, or at least fix for the lower population with more than 8 slots.
In my experience, what happen is after you select (save on server…) a mini on the 9th character, it will reset back the oldest logged-in character, un-checking the box basicly (loosing the paramater). Either the system isnt made to remember more than 8 mini, or something is broken over that limit, since theres no word on the matter from them, we will never know probably.
Note that before HoT release, it was even more messy than what we have now, so they tryed to fix something at that point, but wasnt enought, or just limited-to-8-slots-fixing.
Please fix minis once and for all, its been time!!!
