According to the Kodi Wiki, Kodi 20 starts supporting WebVTT subtitles. But our LibreELEC 11.0 (2023.03) setup does not seem to be able to play them back. Kodi does show the subtitles in the list to select them, but no text appears on the screen. However, the same subtitles converted to ass do work.
Sadly I don't have any other otherwise working Kodi setup to evaluate if this is a LibreELEC or a Kodi issue. I can privately share my specfic file if someone else wants to test this (is a trailer of a movie). However, the WebVTT subtiles work in mpv on my computer without it throwing any errors.
For sake of completeness: LibreELEC 11 running on a RockPI 64, video is 4K HDR (however I hope these have nothing to do with that issue)