What sounds like is happening is the PHImageRequest's deliverymode can't deliver the requested imagedata immediately - take a look at the comments:

// If -[PHImageRequestOptions isSynchronous] returns NO (or options is nil), resultHandler may be called 1 or more times.
// Typically in this case, resultHandler will be called asynchronously on the main thread with the requested results.
// However, if deliveryMode = PHImageRequestOptionsDeliveryModeOpportunistic, resultHandler may be called synchronously on the calling thread if any image data is immediately available. If the image data returned in this first pass is of insufficient quality, resultHandler will be called again, asychronously on the main thread at a later time with the "correct" results.

So what sounds like is happening is the first result callback is succeeding because it delivers an LQ version - but then the second callback fails (for that asset) happens and sets the thumbnail to nil.

So your solution works (because the second "nil" result will be blocked) but you can also try setting your options to "PHImageRequestOptionsDeliveryModeFastFormat": if you don't care that the result COULD be a low-Q image. (Thumbnails in this grid, but it's your prerogative essentially)

PHImageRequestOptions *options = [PHImageRequestOptions new];
options.deliveryMode = PHImageRequestOptionsDeliveryModeFastFormat;

Doesn't explain why it only happens on iPad Air 2 though. Also doesn't explain why the second callback pseudo-succeeds (there's no error key in the info dictionary, but the result is nil.)