When a new cue is emitted from the parser that has the same contents and similar start time to an existing cue, it will be de-duplicated by dropping it. However, some encoders will split cues across WebVTT file boundaries and in this situation, valid cues are dropped.
Pull request: https://github.com/WebKit/WebKit/pull/996
Committed r294854 (250988@main): <https://commits.webkit.org/250988@main> Reviewed commits have been landed. Closing PR #996 and removing active labels.
<rdar://problem/93939179>