Most likely it's due to the different lists you can add or use on NextDNS. I also have issues with captive portals (I run a number of lists on NextDNS) and I just flip it off and on when I need to.
I just checked, and I don't use any lists, except for an allow list I just started with captive portal domains. Eg .aainflight.com, .captive.apple.com, etc