i inspecting someone today who happened to have a rank 25 legacy on their legendary.
I know they upgrade to max rank of 9 and then +1 rank per starlit crystal you put on making it a 12, but how do you get to 25?
and yes i double checked it is a "Rank" of 25, not tier, or stat effects...etc...etc and he had no crystal upgrades.
Sounds like a bug to me. Do you remember what legacy it was, or if the value was also out of bounds?
Some of the default legacies, like "Shield Use Rank" or "Tactical Healing Rating" and other stuff like that, seems to actually have a continual span of ranks where a higher level items "rank 1" is actually a "true rank 14" or something like that.
Next time you see something like that, check what legacy it is! It probably was the basic "inbuilt" legacy of the item.
is was from a burglars legendary class item tool, a major legacy with "+688 Off-hand Critical Chance".