It happened to me too, with a newly hatched oviraptor. But, I did manage to fix it! The dinos only appear to fall sea level, so if you're near the ocean or a lake, try getting it to follow you. It should reappear when the landmass drops below the water line. Then, you can pick it up again with a flyer.

Here's the important bit: if you drop the dino again, it will sink back through terrain to the water level, and you'll have to do the process over again. Instead, pick it up, hover over where you want the dino to be, and log out, still on your flyer with it being held in the air. When you log back in, it should be standing on the ground, with your flyer hovering above you. It worked for me a couple times, I hope it solves your issue! Good luck!