my guess, it’s the same issue as I had with my Meraki agent,
The solution was to put a init.py file like this
OMD[cmk]:~$ cat local/lib/python3/cmk/plugins/cisco/__init__.py
#!/usr/bin/env python3
from pkgutil import extend_path
__path__ = extend_path(__path__, __name__)
in each sub directory under
~/local/lib/python3/cmk/plugins/
Cheers
Thomas