If you are using image-based subtitles (PGS, VOBSUB), ensure they are muxed-in.
If you for some reason you absolutely must use external ones, use SRT-formatted, text-based ones. If you can’t find one online, make it yourself: Introduction: convert image-based subtitles to SRT with Subtitle Edit
A standard tool to remux files is MKVtoolnixGUI.
There is a command-line version available as well, so you can incorporate it into your own scripts etc.
