After a little testing, this bug seems to happen after ranking up champions.
so after tagging a champion and then ranking up any other champion in the tavern, it seems some tags get reset or not saved.
I've had similar problems, though I hadn't figuerd out a link to ranking up. I also run into situations where the thumbnail portrait in the champion list shows a wrong one even while simultaneously the large champion image will show the correct role tag.
OS: Windows 10
I have experienced this problem since the feature was introduced. It happened again today on multiple champions.
I cannot reliably reproduce the problem. It presents itself as a random problem which leads me to believe there is some memory corruption issue that triggers it.
I have NOT experienced the problem on the mobile/android app.
