Bug with some .srt files (preview not working)

I've a small problem with some .srt subtitle files that don't display in the preview, unlike others .srt files— the other subtitle formats supported by Unfold open without any problem.

To try and understand the problem, I opened some of these troublesome files in a text editor (like BBEdit). At first glance, I don't see any issues with it. If I save a copy of it, without modifying its content, this new file then previews normally, unlike the original file.

I therefore suspect an issue with the initial tool used to create this subtitle file, an element, metadata, etc., that prevents Unfold from handling it, but not a text editor, which is more flexible for open this type of file. And by making a copy, this missing/malfunctioning element/metadata is then added/modified, allowing Unfold to handle it.

Understanding exactly what's wrong, however, is beyond my capabilities.