I just played a game in Arena mode where two bugs occurred. One, my opponent had a secret out. I played a minion, and it was not triggered. When I played Rattlegore next turn, it triggered Mirror Entity. Later, my opponent had a 1/1 Rattlegore from Gift of Luminescence. When it died, instead of getting -1/-1 and becoming a 0/0, it left an 8/8 behind, losing me the game.
The first depends on the board state. Was their board full? If so, mirror entity won’t trigger.
The second is not a bug. Minions lose all buffs and debuffs at the moment of death.
My opponent had 1 minion at the time. The second is definitely a bug, because 1-1=0, not 8
I’m not sure about the mirror entity thing but the rattlegore deathrattle is not a bug. Rattlegore doesn’t actually get -1/-1 of its stats when it dies, it would die infinitely because it would die at zero health and ressurect at -1. How rattlegore actually works is the original has Deathrattle:summon an 8/8 rattlgore with deathrattle summon a 7/7 rattlegore with deathrattle summon a 6/6 etc. When they cast gift of luminance it got this deathrattle, this is intended.
Did you play the minion, or was it summoned by an effect (battlecry, recruit, etc)? Mirror Entity only triggers on played minions.
Rattlegore obviously does not work that way. A dead minion has zero health, right? So the deathrattle would resummon it at -1 health, it would immediately die, it would resummon again at -2 health, etc. Nobody expects it to work that way.
Buffs/debuffs are cleared and then it is resummoned.