Starting with mimic/RBD clone format 2, snapshots don't require protection to be cloned. What it happens under the hood is that snapshot with clones, when deleted, are moved to the trash namespace (ID=2). However, these snapshots are still listed when using img.list_snaps().

Reproducer:
Update cluster setting: ceph osd set-require-min-compat-client mimic --yes-i-really-mean-it
Create RBD image: rbd create test_img --size 10G
Create RBD snapshot: rbd snap create test_img@test_snap
Create RBD clone (format 2): rbd clone test_img@test_snap test_clone
Delete snapshot: rbd snap rm test_img@test_snap
The following code:

import rados, rbd

cluster = rados.Rados(conffile = '/etc/ceph/ceph.conf', conf = dict (keyring = '/etc/ceph/ceph.keyring'))

cluster.connect()
ioctx = cluster.open_ioctx('rbd')

# Opening an RBD image with a cloned-and-then-deleted snapshot
img = rbd.Image(ioctx, 'test_img')

# This works
>>> for snap  in img.list_snaps(): print(snap)
{'id': 33, 'size': 1073741824, 'name': '0a0daa4f-092d-4784-98e5-a87921d45aba', 'namespace': 2, 'trash': {'original_name': 'test_snap'}}
{'id': 34, 'size': 1073741824, 'name': 'new_one', 'namespace': 0}

# This breaks
for snap  in img.list_snaps(): img.is_protected_snap(snap['name'])
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "rbd.pyx", line 2774, in rbd.requires_not_closed.wrapper
  File "rbd.pyx", line 3668, in rbd.Image.is_protected_snap
rbd.ImageNotFound: [errno 2] RBD image not found (error checking if snapshot b'test_snap'@b'0a0daa4f-092d-4784-98e5-a87921d45aba' is protected)
The solution proposed by Ilya is to filter out images belonging to namespace == RBD_SNAP_NAMESPACE_TYPE_TRASH.