IOS 15 Captive Portal Auto Pop not showing post session timeout

With IOS 15 firmware version on the iphone our customers are seeing the below issue:

1. Connect Iphone to a Captive Portal enabled(Hotspot) WLAN and Auto pop up for sign in shows up.
2. Let's say your session timeout for this Captive Portal usage is 30 minutes then post 30 minutes the Access Point sends a 802.11 WiFi De-authentication/Disassociate packet to the Wireless Client(Iphone)
3. Once the Iphone is disconnected it reconnects immediately to this same WLAN but it doesn't trigger any Captive Portal Detection and no Auto pop is seen on the iphone.

Due to the above issue the customers are facing sign in challenges who are using public WiFi where they need to sign in again on the captive portal if they wish to continue using the WiFi after there limited session times out. They need to disconnect from the WiFi manually from the iphone and reconnect to get the Auto Pop up working to get to the Captive Portal Sign in page.

Previous versions of IOS and Android devices work as expected and immediately pop up the sign in page post session timeout.