Just happened to me too.
Steps to reproduce:
1. Deep-Cavern Bat exiled my Sagu Wildling a couple turns before.
2. I casted Obliterating Bolt targeting the Deep-Cavern Bat to no reaction from either me or the opponent.
3. The card does seem to weirdly be "present" in the hand (i.e. it makes hovering sounds, and when you draw new cards there's a blank space besides it), and in the opponent's exile zone it shows as "castable" from its blue border, but neither through my hand nor the opponent's exile zone could I cast Sagu Wildling, neither as a creature nor as a sorcery-omen.

Just happened to me as well. You can see where the card should be in the 2nd image, and I could click and drag the invisible card and it'd highlight mana, but I couldn't play it. Lost me the game...

Had the same issue. My opponent played Deep-Cavern Bat exiling my Bristly Bill. Then in a following turn he attacked with it which I blocked with my Waterwind Scout which killed the Deep-Cavern Bat, but instead of returning my Bristly Bill to my hand it kept it attached to the Deep-Cavern Bat as it went to the graveyard and created an invisible card in my hand. I could not in any way play my Bristly Bill after that.

Same here too

Same