I'm seeing issues with WebVTT subtitles drifting out of sync with the video. If I seek, they get back in sync, but drift out again. The timestamps in the webVTT files all seem correct. Here is an example:
Variant List m3u8 URL: http://content.uplynk.com/3370fb9a2d6645488f833c9324c75383.m3u8
The subtitle "of motion, flexibility, stabilization and core body temperature" should show up at about 11 seconds into the video:
WEBVTT
X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:721920
00:00:02.023 --> 00:00:03.067 line:85% align:left
A mobility workout increases range
00:00:03.108 --> 00:00:07.114 line:85% align:middle
of motion, flexibility, stabilization
and core body temperature
The stream starts with MPEGTS = 0, so MPEGTS 721920 is 8.021 seconds. Adding the 3.108 timestamp for the caption from the .vtt file, the caption should show at 11.129 seconds. But instead, it shows at 17 seconds when played back on iOS (I've only tested iOS 8.1 and 8.3). Captions show correctly when played with QuickTime 10.3 on OS X.
Is there something wrong with my .vtt files? Or is this an iOS bug?