You’ve not stated what package you used to create the mkv files. I am familiar with makemkv, and it is excellent with forced subtitles, but it’s a tricky area, as some discs have forced subtitles embedded in the full subtitle track, and other titles have dedicated separate subtitle streams for the forced subtitles (the Marvel Avengers films use this method a lot, also several X-Men blu rays). So if there are multiple English-language subtitle tracks, extra care may be needed.
A nice feature of mkv files + Kodi is that a subtitle track can have a default flag set for it to tell Kodi when to enable a forced track. In Kodi, I have the subtitle option set to “Original stream’s language” in Player Settings > Language, and with a properly prepared mkv file, behaviour is identical to blu ray discs. Here are some examples:
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Single main language subtitle track. In makemkv if you check the options for both the full subtitle and the forced subtitles, makemkv will figure out if there are any embedded subtitles, and if there are, it will write two subtitle tracks to the mkv file, with the forced track flagged as default, and Kodi will spot that and show them. If there are no forced subtitles, you get a track with the main subtitles, but importantly, makemkv will not set the mkv default flag - so Kodi will not show the subtitles by default even though they are in the file. This is very nice. I think GoT uses the forced approach embedded in the main track. If these have been extracted but the wrong mkv flag is set, you could remux the file and change the flag.
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Dedicated separate track for forced subtitles. You’ll need to do some pre-work in this case to figure out the track. Just popping your disc into a blu ray player is an easy way to do that, you will usually find the player has switched to that subtitle track even if it’s not showing subtitles as enabled. Enabling advanced options in makemkv is helpful here as you can manually set the mkv default flag for the target track even if you are also importing other subtitles. This can also be necessary if the entire track is forced, rather than specific subtitles within it, and makemkv does not find any “forced” subtitles, as the blu ray is configured to push the subtitles via the language menu option.
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DVD subtitles. Not the same thing as forced, but for completeness, I like my subtitles to be available if required. So for DVDs, it’s possible to check the option for the subtitle track in makemkv and then de-select the default status for the mkv flag in the makemkv advanced settings. That way the subtitles are in the mkv file but Kodi will not show them unless you ask it to.
Hope this helps. This is an area where a little experience goes a long way. I’ve not come across a blu ray player that handles mkv subtitle flags as seamlessly as Kodi, but if anyone is aware of one I’d be interested to know. My brother can’t get his Oppo 203 to do this seamlessly, he has to activate the subtitles even though the default flag is set in the mkv file.