What I generally do is inspect each sub stream and turn the forced on and off to see whether there are any forced subs in that stream.
If I can find no forced subs, I then go looking for a stream that contains only the subs that I want. This is the usual scenario when Fab can't find forced subs.
If all that fails, I go to opensubtitles.org, download a SRT subtitle and inspect it with a Subtitle Edit, and if correct, mux it into the Fab produced MKV withMKVToolnix.