I have made a successful rip of the film. Playback and audio is perfect except missing forced captions. I checked the forced caption box was selected when ripping but cannot get them to display. Bit of a problem during the Ape sign language scenes where you cannot read their lines.
It's not that easy with forced subs.
Some movies use actual forced subs - which means, some of the subtitles in a subtitle track are flagged as "forced", others not.
With the option checked, CloneBD will create an additional track for the MKV with the forced subtitles only.
In your case here, it's probably another quite popular "technique": the subtitles are in a separate subtitle track and there are no forced flags.
Then you'll have to select that track for display.
The only way to find out, is to use the preview player in CloneBD and check the available subtitles.
If there's one, where the lines only show up, where you'd expect a forced subtitle, that's your candidate.
Unfortunately, there is no good way to do this automatically, because CloneBD would have to scan the whole movie and check for those forced flags.