How to implement OAuth (authorization code flow) for mobile-apps using deep-links as redirection URL

Have you tried some custom irule as I am not an oauth expert by no means but maybe try configure the redirect as an irule as the example below:

https://community.f5.com/t5/technical-forum/original-url-redirect-after-apm-authentification/td-p/262148

There is also an iRule option to be attached under the OAuth Server  but there is not a lot of info for it:

https://community.f5.com/t5/technical-forum/using-custom-oauth-client-irules/td-p/254118