subtitles with WebVTT format disappear after playback for a while on iOS12

Hi there,


After updated to HLS V4, we found two problems in iOS devices with our WebVTT format subtitles:

1) iOS11.4.1 subtitles display does not sync with audios (some devices are ok, some are not)

2) iOS12 subtitles donot display or diappear afer playbakc for a while. (most of our devices with iOS12 have this problem)


the WebVTT as follows:

--------------------start sample-------------------------------


X-TIMESTAMP-MAP=MPEGTS:1250367422,LOCAL:00:00:00.000

00:00:00.000 --> 00:00:07.978 line:80% position:20%
-Du likner pa en mus.

00:00:00.000 --> 00:00:07.978 line:84% position:12%
-Det er fordi jeg er en mus.

00:00:07.978 --> 00:00:10.000 line:84% position:12%
Det er i dag! Det er i dag!



WEBVTT
X-TIMESTAMP-MAP=MPEGTS:1251267422,LOCAL:00:00:00.000

00:00:00.000 --> 00:00:01.618 line:84% position:12%
Det er i dag! Det er i dag!

00:00:02.618 --> 00:00:05.498 line:84% position:30%
Det er i dag!

00:00:07.018 --> 00:00:09.338 line:84% position:30%
Det er i dag!

00:00:09.498 --> 00:00:10.000 line:80% position:7%
-I dag er alltid i dag, George.

00:00:09.498 --> 00:00:10.000 line:84% position:15%
-Kan jeg ikke fa bli med?


-------------------end sample--------------------------------


we tested the follwoing sample flow is OK:

http://sample.vodobox.com/planete_interdite/planete_interdite_alternate.m3u8

Replies

Another thing, we found subtitles displayed abnormal in Safari12 of problem iOS12 devices too: display just for a while; display one too long; display many together..., however, it displayed normal in Quicktime player and Safari12 of macOS 10.13.6.

We have the same issue. It's working fine on iOS 11, but on iOS 12 we get the same issues with subtitles that you are describing.

Thanks very much for your information. Apple DTS has reply to me that this is a bug in iOS12 and has been fixed in iOS12.1.