Did you get some of that cold weather we got here recently? A sudden cold snap can chill some brood and kill it, quit often drones scattered around the outside edges of frames.
I had one hive haul out quite a few the other day, with lots more bits of pupae on the sticky board. Too much brood for them to cover when the air temp dropped to near freezing after a couple weeks of warm weather.
I wouldn't worry unless it continues.
Got to 49 Friday 45 last night Prediction is 44 tonight then back up to middle fifties
I thought it could be the cause just seemed weird to only happen in one hive it's the second strongest of my 4
Nothing seemed odd when I went into the hive today
Interesting, I was wondering myself. I found a dozen or so dead immature drones outside my hives recently. Looked like chalk brood but I only saw drones. Its been very cold here lately, 25 degrees and a foot of snow after a week of low sixties.