The track may use images or its own styling, so it may not respond like an ordinary text subtitle.
Why this happens
Not every subtitle is stored as a plain line of text that the player can restyle. Some formats contain pictures of the words, while styled text formats can carry their own presentation information. This explains why a font change can affect one track and leave another looking exactly the same. It does not necessarily mean the setting was ignored or the whole application needs reinstalling. The type of track and the player’s styling rules need to be considered together.
Kodi and VideoLAN both distinguish text-based subtitle settings from image-based subtitle behavior in their documentation. Begin by comparing another track or item rather than changing every display option. If the player provides an override for styled subtitles, read its description before using it; an override can also alter placement and layout. A separate text track may offer more presentation control when the source supplies one. Keep the track format or label with the result, and remember that source-provided images cannot be converted into clean editable text just by choosing a different font family.
A useful way to work through it
Compare whether the font setting works on another text subtitle track.
Identify an image-based or styled track if the player exposes that information.
Use a documented style override only after preserving the original preference.