We found this strange behaviour majorly with iPhone's where the devices are not initiating to authenticate to the captive portal network. This behaviour happens when the session time for the network access is expired and the client needs to re-authenticate to gain the network access. All client needs to authenticate/re-authenticate to gain network access.
Once the session time is expired, the client remains connected to the SSID but fails to initiate the TCP connection to the captive portal server to get the splash page to re-authenticate himself.
The detailed observation is that (after analysing the packet capture) once session timeout is expired, AP disconnects the client and client connect back to the SSID immediately. But the TCP connection to get the splash page is not initiated. Once the TCP connection to the server is established (by manually- clicking on any link), the splash page appears in the client.
While this seems like triggered by client behaviour (iPhone). This behaviour is seen in IOS (iPhone's, MacBook,iPad's) and Android devices (version 9). This is not seen in Windows Surface and Android devices (running any version less than 9).