Albumen paper curls. That is why virtually all albumen prints are mounted to something stiffer.
If it curls in the tray while it's floating in the albumen, just try to keep the corners in contact with albumen without letting albumen get on the back of the paper. You can also leave more of a margin to cut away when it dries.
Steaming the paper before floating it in the silver nitrate keeps the paper pliable and helps reduce curling in the tray.
I haven't tried steaming the paper before floating it in the albumen, but it might help.