AI failed to cast Grisly Sigil (SNC) but still paid and sacrificed; card stopped existing

The AI had a Riveteers Requisitioner out, and I had nothing. They tried to cast Grisly Sigil, paying the mana and sacrificing their creature for casualty. The log said "Couldn't add to stack, failed to target", but their mana was spent and the creature was gone. Additionally, Grisly Sigil wasn't returned to hand and simply disappeared (their total card count in all zones is now 39)!

This shouldn't have been possible in the first place, as they shouldn't have had the opportunity to pay costs including the sacrifice before choosing targets. It would have been possible for them to target their own creature, then sac it as part of the cost, thus making both the copy and the original fail to resolve for missing targets.

Not sure why the card would have disappeared, unless it decided it couldn't rewind due to the sacrifice.

Daily (current) snapshot from yesterday, Windows.