Sometimes it doesn't copy the unit with a spy status/on deathblow, sometimes using it even locks your cards entirely. You can't play them anymore, they don't react to clicks and anything else.
1. Executioner. It had both Bounty and Spy on it, too, maybe that's relevant. Cards started acting up after that, becoming invisible after being played, but the Exec itself wasn't even copied
2. Ferkart. Actually was done by my opponent, and my cards became unresponsive, so I was forced to concede. Also caused some weird "invisible cards" stuff.
3. CdG on Terranova with the spy status. Same stuff as the previous example, except it seemed to lock my opponent's hand this time. Seriously doubt they just roped the last 3 cards into discard.
4. Occasionally CdG just doesn't trigger Joachim (and he wasn't purified) I wasn't counting, happened probably two or three times to me, and I've seen one opponent suffer from this bug.
I didn't spot any special pattern of any kind, except that it seems to happen more in Assimilate mirrors (but that kinda makes sense, more de Grace, more chances to trigger the bug)