Captive Portal doesn't do anything when I connect a Windows 8.1 laptop to IIAB 6.7/master with captive_portal_enabled: True

Same sad story as when WiFi-connecting an Android 8.1 device (to IIAB 6.7/master) as described at #1323.

The client machines (Windows 8.1 and Android 8.1) were each connected over WiFi to 2 different IIAB 6.7/master servers (Raspbian & Ubuntu 18.04) to reconfirm the bad news!