My Sigil just turned into a Pink Robe that was named "Woopsie", sort of like how a 9mm Revolver is a Pink Robe just renamed. All I did was take it out of my bank and this happened. Putting it back into my bank turns it back into a Sigil, but taking it out just turns it to a Pink Robe again.
It's because you're withdrawing it as a noted item. Put it back in the bank and withdraw it unoted and it should be fine.
Thanks a bunch!
Anytime Anytime! ;D