We have experienced the same issue in our app i.e. both long and short Firebase Dynamic URLs were not launching the expected screens in our app when opened from the Facebook Android app. After testing we found out that Facebook App's built-in web browser is resolving the Firebase Dynamic link URLs into actual deep links i.e. it is stripping all data from the Firebase Dynamic link URL except the link field before forwarding it to our app. For example, it was resolving:

https://xxxxx.page.link/XXXXX

into:

https://www.xxxxx.com/playlist/34

The fix was to modify our Activity that receives Dynamic links and routes them, to expect this. Below is the sample code that does this. It should go into the Activity’s onCreate() method or be called from the onCreate() method: