Based on my previous experience and this answer, Pokémon that you left in friendly arenas that got beaten would return to you with 1 hp, thus allowing to heal them directly with potions.
However, today, the two Pokémon that I left in a friendly arena returned to me after some time with 0 hp, which means that I now need revives before healing them.
Has there been any word from Niantic / any external confirmation that this mechanic has been changed, and that now beaten Pokémon always come back to you with 0 hp?