When an enemy flying unit dies, the first time you see it again after it respawns, it will raise out of the ground until it reaches its flying height again, so as a result, will essentially be invisible for the first few seconds

Usually this affects enemy couriers, but will also affect enemy Batrider and Wyvern if they die during Firefly or Arctic Burn

Example:
https://gfycat.com/enormousdisastrousdeviltasmanian