I just had this happen in a game today. I was playing with Bandersmosh and it transformed into Flobbidnous Floop while it was in my hand, which then transformed into the last minion I played. The following turns it didn’t change into any new legendaries. It just kept changing into the last minion that I played, keeping the Flobbidinous Floop text. Not entirely sure this is a bug, but it seemed like one