When playing HLS, can't show external subtitle

I need paly m3u8 file, and add subtitle, but I found when I play my m3u8,then add an ass subtitle file, the ass file can be added but can't show ! I don't why? When I play a mp4 file,the same subtitle file can show ! I try to use VLC client 3.0.8, the result same as use VLCKit, already can't show! Please Help me!

I using VLCKit 3.3.9, below is test file:

9596726-5937132-4.m3u8 SUB.ass

Edited Jun 29, 2020 by Jean-Baptiste Kempf

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