I think that means that either
ex1 is not unique and you need to use a prefix like
.../live-status/modprefix:ex1/..., or your device model has not been properly loaded by the system.
You might make things a bit easier for you if you use so-called high-level Python API and maagic API; I think you are using NSO - in that case check the corresponding chapters in NSO Development guide.
If things are set up correctly, something like the following should return
from ncs import maapi
with maapi.single_read_trans('admin', 'system') as tp:
root = maagic.get_root(tp)
device = root.devices.device['netconfd']
'ex1' in [c._name for c in device.live_status._children.get_children()]
(Note that the code is using attributes
_name that should be used only for debugging.)