I see it's been a long time since the last post, but I might have a theory
(or it could be another bug)
I had an arena deck with some unupgraded Shards of Sacrifice (under 6)
I then went to upgrade one
The original unupped shards were still there, so i could put in another upgraded shard
PS. I might have gotten an upgraded spin from a farming arena silver, but that might just be me accidently beating Decay