I'm able to reproduce this issue by connecting an Ethernet cable, and then disconnecting as soon as the "Connected" notification appears. I can also cause captive network assist to incorrectly appear by connecting to a WiFi network, and disconnecting as soon as the notification appears. What appears to happen is that the NetworkManager up event is triggered, which causes an attempt to connect to the internet. If the connection fails, captive network assist appears, assuming the user needs to login to a captive portal to connect. Between the up event and trying to connect to the internet, the connection goes down. This unexpected event causes captive network assist to appear in error.
I'm able to reproduce this issue by connecting an Ethernet cable, and then disconnecting as soon as the "Connected" notification appears. I can also cause captive network assist to incorrectly appear by connecting to a WiFi network, and disconnecting as soon as the notification appears. What appears to happen is that the NetworkManager up event is triggered, which causes an attempt to connect to the internet. If the connection fails, captive network assist appears, assuming the user needs to login to a captive portal to connect. Between the up event and trying to connect to the internet, the connection goes down. This unexpected event causes captive network assist to appear in error.