1. Tame an already-in-the-world creature
2. Leave them sit for a while
3. Leave them sit and go away from them
4. Return and try to unsit them

I tamed a Royal Red and the dragonfruit drake and left them sit while I went to my base (which was almost like 1000+ bocks away) and after returning for them, I tried to unsit them, but they just stayed in the sitting position, but slid on the ground. They weren't able to teleport to me and I couldn't even ride them, because they just kept going in the same direction they were facing. Also good to mention, I use the waystones mod, and after I teleported thanks to the waystone to them, they were bugged out like this.

Also wanted to mention, that this also happens with Silver Gliders as well, they keep flying into the same direction they are facing the entire time, not teleporting to me.