When you created this subvolume, your subvol root was mounted at /. So when you created the new subvolume /shared, it was created inside the subvolume at / (i.e., root), with the name shared. While I get what you are trying to do, please note that all the subvolumes listed in your btrfs subvolume list command are not “top-level” subvolumes, even though it may look like that. The designation of top-level subvolume in BTRFS is used for the initial subvolume with ID 5, see the subvolume documentation.
To create another subvolume under the one with ID 5, you need to mount the ID 5 subvolume somewhere, and then create it at that mountpoint:
$ sudo mount -o subvolid=5 UUID=5abde8a9-aba0-4f70-a877-3fcedd20905e /mnt
$ sudo btrfs subvolume create /mnt/shared
$ sudo btrfs subvolume list /mnt/
Mounting subvolume ID 5 is the default of a BTRFS volume. I just wanted to make it explicit in the mount call above, in case you or the installer had changed the default subvolume.