Was fighting the second boss of the tavern brawl, and my Sylvanis died. The deathrattle targetted a sandpile and made my hero immune, as you would assume from the text. It also kept the boss immune intermittently, while he had no sand piles. Damaging the sandpile on my board (not killing it) seemed to remove the immune effect on the boss for that turn.
Playing the card (Pile of Sand) from hand (after e.g. copying it) and then buffing it so it can attack, doesn’t allow it to attack, unlike the one gained from taking control of one played by Buried Finley which can attack after being buffed. So strangely enough the same card acts in two different ways.
Okay, so 3 turns after I played the copy of Pile of Sands from my hand, and with it still in play. Buried Finley lost his immune after I once again killed his Pile of Sand (as I had done on the previous turns as well). So following that turn, Buried Finley loses his immune when I kill his Pile of Sand, while my hero retains her immune from my own Pile of Sand. However it still can’t attack .
I experienced a different bug. I used thoughtsteal to get a spell breaker from his deck. I silenced the only pile of sand that he had on his side, however, he was still immune until I killed the silenced pile of sand.
Yeah basically there are two parts, piles of sand grant immune but any pile of sand being present still stops you from digging up Finley. Can see how that could be a little confusing.
https://us.forums.blizzard.com/en/hearthstone/t/finleys-boss-encounter-bugs-with-mind-control/13552
The Pile also seems to have a CANT_ATTACK flag, which gets toggled off under certain conditions: https://us.forums.blizzard.com/en/hearthstone/t/pile-of-sand-giving-both-heroes-immune-odd-behaviour/13555
https://www.reddit.com/r/hearthstone/comments/czxm2e/pile_of_sand_still_gives_the_enemy_hero_immune/
Dev response, but vague on whether all of the behavior reported is intended: https://www.reddit.com/r/hearthstone/comments/czpowg/finley_bug_in_tavern_brawl_another_bug_i_saw_was/ez0nr9i/