The issue is specific to release 21.4.L4. In this version the last VN associated with a Logical Router can never be removed.
After saving it from UI workflow, the VN never gets disassociated.
When you try to edit a Logical router from the UI, the last VN associated with a Logical Router can never be removed
There is no error messages, however VN never gets disassociated from the LR.
The following error message appear in the config api logs
File "/usr/lib/python2.7/site-packages/vnc_cfg_api_server/resources/logical_router.py", line 249, in _check_connected_networks
vn['virtual_network_properties']['mirror_destination']):
KeyError: 'mirror_destination'
The issue is fixed in 21.4.L5