Please, your question is very specific, it can take some time for somebody who knows the answer to pass by. Also, you might get better luck on Stackoverflow with a question like that ?
I'm not familiar with Ionic, but you probably need to find a way to open the Facebook modal inside the webapp. If it opens in the Device Browser, there is no way for Facebook to issue a redirect to the forum inside the webapp. Isn't there a setting to configure which URLs schemes should be kept in the webapp instead of opening it in an external browser ?
Or, alternatively try to do the OAUTH Facebook login manually inside the app (with an Ionic plugin if that's a thing ?) and then redirect that token to the forum.