It’s been a problem forever. The necro sub has talked about this forever now. It’s the built in cleanup routine. It’s especially bad in hordes because your minions will make corpses off screen and the routine will clean up the ones on screen, leaving you zero usable corpses.
I believe I S3 we figured out the corpse cap is around 12 before it starts cleaning them up.
Doubt it will ever be fixed since blizz is in coast mode until voh.