Sometimes subtitles aren't displayed by default

[haru2187]

Maybe because MKV has flags for default and forced subtitles that TS has not.

This page says "VLC can read subtitles for the following media formats: DVD, SVCD, OGM files, Matroska (MKV) files". Certainly it doesn't include ts files.
https://wiki.videolan.org/Documentation:Subtitles/

Is it possible to request that ts files be included as well?

[Rémi Denis-Courmont]

Note that TS subtitles need to be encapsulated inside the TS multiplex. You can't use external subtitles. So that's why it's not listed there.