This is still happening with IpadOS 17.2 - the iPad will associate to the WiFi correctly, and obtain a DHCP lease, but because it can’t ping something on the internet from behind the captive portal, safari will never get the memo that it’s connected and it won’t even attempt to load the captive portal page.
I am sure that somewhere this design made sense to be “helpful” to end users, but it’s mildly infuriating when it only gets in the way of trying to connect. Meanwhile, I’m connected and able to access the captive portal from my iPhone sitting a few inches away. So this seems to be an iPad-specific problem. Disabling private MAC doesn’t help.
it’s also highly annoying when it tries to be “helpful” in this manner when connecting to a WiFi network that has no intention of providing internet, such as for IOT and device control.
at the very least, give the user the ability to turn this nonsense off. Or let us identify networks that require captive portals or that don’t provide internet.