Auto-activation of secondary VPN prevents captive portal detection
When I set up a VPN to always be activated as a secondary connection to, e.g., the Wi-Fi interface, NM no longer issues requests to detect captive portals and the client will be stuck in the captive network unable to establish the VPN connection.
A solution would be let NM detect captive portals in any case and hold of secondary (VPN) connection as well as all other outbound traffic until the captive network is "open".
I was testing NM 1.22.10 on Ubuntu 20.04.