Working Android 10 with Firefox:
[29/Jul/2020:15:36:54 +0100] "GET /oauth2/auth?redirect_uri=com.testing.talk%3A%2Foauth2redirect&client_id=9323-f1c6885eba5e&response_type=code&login_hint=ghenry%40testing.co.uk&state=g6Epg6capgPQd3PvwVH7wA&scope=openid%20offline&code_challenge=HzONZ6WZVFTLQgRmpzFXuUeKDnm2OuuykBYIgez_uSU&code_challenge_method=S256 HTTP/1.1" 302 107 "-" "Mozilla/5.0 (Android 10; Mobile; rv:68.0) Gecko/68.0 Firefox/68.0"
[29/Jul/2020:15:36:59 +0100] "GET /oauth2/auth?client_id=9323-f1c6885eba5e&code_challenge=HzONZ6WZVFTLQgRmpzFXuUeKDnm2OuuykBYIgez_uSU&code_challenge_method=S256&login_hint=ghenry%40testing.co.uk&login_verifier=32d29a635cf74e3eabe6124ff9943ae7&redirect_uri=com.testing.talk%3A%2Foauth2redirect&response_type=code&scope=openid+offline&state=g6Epg6capgPQd3PvwVH7wA HTTP/1.1" 302 111 "https://portal.testing.co.uk/login?login_challenge=5f3bc29480944df69ff57136d1f13ac6" "Mozilla/5.0 (Android 10; Mobile; rv:68.0) Gecko/68.0 Firefox/68.0"
[29/Jul/2020:15:36:59 +0100] "GET /oauth2/auth?client_id=9323-f1c6885eba5e&code_challenge=HzONZ6WZVFTLQgRmpzFXuUeKDnm2OuuykBYIgez_uSU&code_challenge_method=S256&consent_verifier=e94b16a44e8e49808487769d72123317&login_hint=ghenry%40testing.co.uk&redirect_uri=com.testing.talk%3A%2Foauth2redirect&response_type=code&scope=openid+offline&state=g6Epg6capgPQd3PvwVH7wA HTTP/1.1" 302 - "https://portal.testing.co.uk/login?login_challenge=5f3bc29480944df69ff57136d1f13ac6" "Mozilla/5.0 (Android 10; Mobile; rv:68.0) Gecko/68.0 Firefox/68.0"
[29/Jul/2020:15:37:00 +0100] "POST /oauth2/token HTTP/1.1" 200 1649 "-" "Dalvik/2.1.0 (Linux; U; Android 10; SM-G970F Build/QP1A.190711.020)"
Android 11, Chrome up to date.
[29/Jul/2020:15:38:02 +0100] "GET /oauth2/auth?redirect_uri=com.testing.talk%3A%2Foauth2redirect&client_id=9323-f1c6885eba5e&response_type=code&login_hint=ghenry%40testing.co.uk&state=IgvZOAhsh8YaCcFw_vnaSw&scope=openid%20offline&code_challenge=ewLWCgvE6rOPKp88r-Oz6y1yMYOq_K_G1Gnr9PR9eXw&code_challenge_method=S256 HTTP/1.1" 302 107 "-" "Mozilla/5.0 (Linux; Android 11; sdk_gphone_x86) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.105 Mobile Safari/537.36"
[29/Jul/2020:15:38:09 +0100] "GET /oauth2/auth?client_id=9323-f1c6885eba5e&code_challenge=ewLWCgvE6rOPKp88r-Oz6y1yMYOq_K_G1Gnr9PR9eXw&code_challenge_method=S256&login_hint=ghenry%40testing.co.uk&login_verifier=61dc1fc565a542eca8ccc8e516ca684d&redirect_uri=com.testing.talk%3A%2Foauth2redirect&response_type=code&scope=openid+offline&state=IgvZOAhsh8YaCcFw_vnaSw HTTP/1.1" 302 268 "https://portal.testing.co.uk/login?login_challenge=06c9086cc29449a3bf055f05d764a668" "Mozilla/5.0 (Linux; Android 11; sdk_gphone_x86) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.105 Mobile Safari/537.36"
[29/Jul/2020:15:38:09 +0100] "GET /oauth2/fallbacks/error?error=request_forbidden&error_debug=No+CSRF+value+available+in+the+session+cookie&error_description=The+request+is+not+allowed&error_hint=You+are+not+allowed+to+perform+this+action. HTTP/1.1" 500 647 "https://portal.testing.co.uk/login?login_challenge=06c9086cc29449a3bf055f05d764a668" "Mozilla/5.0 (Linux; Android 11; sdk_gphone_x86) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.105 Mobile Safari/537.36"