In my work we have a large WIFI guest network (not MikroTik) with around 1600 active users all time.
And there we see the same with all Apple units.
Problem is that they to chache lot of data and when you try to visit the a site it tries to open it from cache and not using portal.
This results in that the portal page does not show up.
Block all Apple units, since they do not follow the wifi standard
Or tell all Apple user to visit tree-four pages that they have not visited before (not in cache). This should bring up the portal page.