What is the crowd's opinion of TEncoder Video Converter?

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Subtitles in MKV files are usually stored as a text stream; the position of the subtitles is dependent on the way the subtitle file is written and the player being used.

If you are trying to burn subtitles onto a video so that they cannot be disabled, use a different tool such as Handbrake, which allows you to burn in subtitles, but unfortunately doesn't allow you to position them. A quick search suggests that TEncoder may have repositioning functionality, but I can't vouch for it.

There may be ways to create a separate video stream with just the subtitles so that they can be toggled and the positioning/styling can be enforced, but I don't think mkvtoolnix can do this.