Not a bug Rival Battle.

[Drakoal]

As I was battling my Rival in the burnt down tower, his Zubat suddenly fainted before my Gasle had a chance to use Sucker Punch, my rival sent in Quilava, but my gasle used sucker punch afterwards on the same turn.
Zubat was not under any poison and I did not use Curse on him either, he had around 15 Health.

 

[Leahkim]

As I was battling my Rival in the burnt down tower, his Zubat suddenly fainted before my Gasle had a chance to use Sucker Punch, my rival sent in Quilava, but my gasle used sucker punch afterwards on the same turn.
Zubat was not under any poison and I did not use Curse on him either, he had around 15 Health.

Did it faint or was it switched out? The new update improved the AI so now you'll see more switching from the npc's.

 

[nilllzz]

at the moment, the trainers can switch out their Pokémon, but that gives your Pokémon EXP right now due to a bug. So this might have been what happened.

 

[Drakoal]

Did it faint or was it switched out? The new update improved the AI so now you'll see more switching from the npc's.

It fainted though as the rival never used it, again I guess its a rare bug, seeing as I haven't seen any other player found it.

But the switch was successfully when he switched Quilava out for magnamite, but the Zubat never came back.