I was able to reproduce this issue on latest Nightly version 92.0a1 (2021-07-22)(64-bit), Beta version 91.0b5, and Firefox Release 90.0.2 on macOS 10.15. I'll attach the evidence on 3 files and I'll change flags accordingly. Severity suggested S3.
Bug 1721736 Comment 1 Edit History
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I was able to reproduce this issue on latest Nightly version 92.0a1 (2021-07-22)(64-bit), Beta version 91.0b5, and Firefox Release 90.0.2 on macOS 10.15 using this URL: https://www.iandevlin.com/html5test/webvtt/html5-video-webvtt-sample.html. I'll attach the evidence on 3 files and I'll change flags accordingly. Severity suggested S3.