In Arena, on PC (Windows 10). I played a Vectus, with no other minions on my board. I’d only had one deathrattle minion die (a Replicating Menace), so expected two whelps, one of which would get that deathrattle. Instead, I got one whelp (the one summoned on the left) with that deathrattle, and no whelp summoned on the right. Possibly(?) related to the bug’s cause: I’d previously played ‘Soul of the Forest’, giving the deathrattle to a Loot Hoarder and corrupted Faire Arborist, to which my opponent played Devolving Missiles and caused them to lose their Deathrattles.
This happened again my very next game, minus what I described in the final sentence. Had a Replicating Menace dead and no other deathrattles, got the whelp on the left with the deathrattle and no whelp on the right. FWIW, I lost the first game with my opponent at 1, so it actually might have affected the outcome.