This looks pretty germane…
I’m sure with a bit more reading you would find others that are still relevant even with newer version of Kodi. As for the logs the part of interest would be in guisettings.xml…
<defaultvideosettings>
...
<showsubtitles>false</showsubtitles>
...
</defaultvideosettings>
Which is the setting that tells Kodi if it should default to enabling subs. This auto enabling that this setting interact with does not apply if forced subs are present as that supersedes this option as it should.
I would suggest to just take one file with no subtitles but a audio track that is properly set in your language of choice and make sure this is either a new file name or location that Kodi has never touched so you know there is no stored information about it in Kodi’s database. Now take a srt and place it next to it with the same name as the video file and open it via Videos>files>[path to video]. When you do this you should at no point ever turn the sub track on/off or change selected audio or sub track or anything else as when you do things get stored in the database that will spoil the test. At this point you have a test bed that you can play around with Kodi’s subtitle settings and rename the srt to see how the settings interact with the various sub tags…
test.mkv
test.srt
test.forced.srt
test.default.srt
test.default.forced.srt
test.nl.srt
test.dut.srt
test.nl.forced.srt
test.nl.default.srt
test.nl.default.forced.srt