when a user enters our app and goes to enter, the app redirects them to an external login page and once concluded they are sent back to our app. However, our login system doesn't have any analytics script and our DPO doesn't allow us to add. This makes us lose any kind of tracking (UTM, deeplink, etc). In the past, if the user was already logged in and used a deep link to enter the app, the deep link worked fine and the user was redirected to a specific screen. But now we have biometric authentication, and because of it the deep link doesn't work anymore, it loses the information about which screen to open.
The only thing I can think on on the app would be to have an "invisible landing screen"? Maybe your deep link takes the user there and gets UTM and other entry tracking, then forwards them to the login screen immediately... it should be invisible to the user, but allow you to get tracking?