a Koalindl fish
 

This fish in North Qeynos, when killed, will only be lootable for a short amount of time. You can keep a target on the corpse, but are not able to loot it. Almost like it fell out of the world.

The issue that the OP presents is not the same as the one linked.

I can also confirm this new bug since the .40 patch. MoB corpses in water fall slowly like they always have (that's a bug?) but instead of landing on the ocean/lake/etc/floor, the corpses fall below the world and are no longer lootable. Confirmed in both the Koalindil pool in Qeynos, and in Kedge Keep.

Seems to be fixed with the server reboot, at least in Qeynos.

Yeah i notice this as well and had made a post ad well about this...this was still happening yesterday