I've come across this problem and I'm trying to pinpoint exactly what the issue is. The file created from MakeMKV has the subtitles as you can see below (VLC top LG bottom) but when I play the file on my LG the subtitle button is greyed out. Power DVD 16 & 18 and DVDFab player all display the subtitles fine so I don't think its a problem with the file.
Anyone else with an LG OLED having this issue? I remember this not being an issue before the 5.x firmware update.
Possible issue with LG player subtitles not displaying.
Re: Possible issue with LG player subtitles not displaying.
None of my "smart" TVs support PGS subtitles. They do OK SRT subtitles, but PGS are not playable. VOBSUB subtitles from DVDs are mixed on support.
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FAQ about BETA and PERMANENT keys.
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FAQ about BETA and PERMANENT keys.
How to aid in finding the answer to your problem: Activating Debug Logging