first lets have some fun and guess how this happend
opponents last troop is poisoned even though he has sturdy trait.
clue (more like to check out hide option to be fair 
) take a look at 3rd troop status effects
and then lets us make a mind is this a bug.
Dragonette was in first place, and the random status effect from it’s spell isn’t checking vs sturdy properly? (you cast twice, once on troop 3 and once on troop 4).
How about Dragonette applied a stun first then poison. Could be doing a series in order instead of all at once - that would allow the stun to negate Sturdy.
That is right it was Dragonette into stun and poison.
To be fair i dont like that outcome, and as soon as stun disapears poison should do that as well.
From using dragonet i can confirm that if it doesnt apply stun, poison will get blocked by sturdy. However stuned troop has no resistance.
No, please. I use that all the time w/ stupid bone dragon.
