Mine disappeared too, no tribelog, nothing.
It also had full food before the update, if it starved then it would be logged in the tribelog.
And on the note about not being able to access the Oasisaurs oasis, it seems to be accessible within a certain radius wherever the Oasisaur spawned/tamed, so if you downloaded it at Green obelisk on the Island, then you'll be able to access it around there. The resources on its back follow the same logic too.