The reason why this item drops off the list is because the track fails to load, its readiness state is flipped and then the UI decides to leave it out (because canBeRendered retuns false when the readiness state has been flipped that way).

So, speaking stictly from a UI-standpoint, this is probably as-designed, but it might make sense to change the design to indicate that the track failed to load. 

For example: Change "French" to "Track failed to load", or something (strawman proposal).

The file it tries to load doesn't exist:
https://mounirlamouri.github.io/sandbox/media/test-sub-fr.vtt 

I think marking this as WAI is appropriate. I fixed the test page.

Thank you for investigating this :)