I found a workaround: You need to disconnect any VPN, etc. and turn off the "Always-on VPN" and "Block connections without VPN" settings. When the captive portal pops up, tap on "Use this network as is". Then you can go ahead and open some HTTP site in Vanadium, I always just use http://captive.apple.com because it's easy to remember. Then you should be redirected to the captive portal, and you should be able to interact with it normally. Completing the captive portal should establish a normal, working connection.
After that you can turn your VPN back on. I would set the MAC address randomization to per-network for Hotsplots until this issue gets fixed. Otherwise you're gonna have to do this every time you reconnect.