After the latest patch, every time I go out for sea monster with my guildmates or by myself, we would take down the sea monster's health down to around maybe 20%, and the sea monster disappears after ramming it one more time. I'm not quite sure if it has happened to anybody else or not since only my guildmates and I have experienced this.
When u r chasing it back to its zone Im guessing? Keep following the direction it went, it will reappear.
Go slowly. With Candidum they sometime reappear slightly behind you already in attack mode and go into dive move when you get back to their spawn point, but seems to take a sec for them to reappear. Between whatever is making them vanish and then the dive mode, ive had them not come back up also a few times if I chased them while breezy sail. Just follow with W alone and they should come back. Same thing happens with ninesharks and black rusts. Candis are the ones that take longest to reappear so its good to go slowly. Extra tip: I also tend to try and not kill them while they are in retreat mode also. The loot isnt always on the same point as the monster body when you do. Might be back at that reload in point and people tend to miss it or think it vanished already.
For sure the coding on them is semi buggy, but still very managable once you get through the trial and error. *learned the hard way and over time*
Thanks @shidoxen for sharing these tips. I'll also follow these situations on to our bug team so they can discuss it with the devs and see if there is anything we can improve.
I just want to update the thread and say that this is bug is still active as of 8/17/2018 Low Hp Sea monsters disappearing.
There is also a new bug where sometimes the loot from the sea monster is non-obtainable, even though you can see the loot pillar, the sea monster is dead, and it is pulsing... indicating the presence of loot and yet it cannot be looted. This started after the most recent maintenance.