After a two year pause, I logged again i to the old account. It looked like I still had everything how I left it. But over time, I started remembering that I am missing some dragons. At first I thought I just remembered it badly. But then I used the search function in Dragonbook.
When I choose to show only dragons that I own, it shows me all dragons I owned at one point, even those that disappeared.
The ones I can still access have the "Find" button in the lower right corner. Those I can no longer access, yet I had them (and certainly didn't sell them) don't have "a Find" or "Sell" button there, and they are still visible when applying filter "owned".
I believe it's possible those were the dragons I placed inside of Dragonarium. I was surprised to come back and find it empty. I think maybe after some update, Dragonarium got bugged?
My user id is
Edit: User ID removed for your own safety. Never show your user id in public.