[Kudux]

I have set the standard settings for subtitles to be at the bottom of the video. I also checked the subtitle settings in video calibration to confirm they are also at the bottom there.
This works great for srt. However, for sub/idx, and some subs that are not baked in video stream but still come included in the mkv container, the chosen position has no effect. Is there any way to force it for these types of subs?

[DaVu]

no

Subtitles which are included in the mkv container can't be changed in position AFAIK. And I only got it managed with srt subs where this is supported.

[Klojum]

I don't think it's about whether subtitles are muxed into a mkv file or not, but sub/idx has its own format that includes (or can include?) the position on the screen.
So Kodi would have to override that position, but I don't think that functionality is in the current Kodi version. Kodi 19 could perhaps is probably a better candidate for it.

I prefer to use external .SRT files, as they are easy to edit whenever spelling mistakes pop up, and there is no fuss like with the other subtitle formats.

[jjd-uk]

The subtitle format sub/itx are graphic type subs I believe and they can't currently be moved, only text subs such as srt can be moved.