I'm using WebVTT metadata tracks to carry SVG content. At a given time, only one cue is active. I'm getting the text using getCueAsHTML but sometimes I have a weird behavior: there are 2 active cues and one of them has truncated text.
This may be related to https://bugs.webkit.org/show_bug.cgi?id=94651. Please retest once that patch makes it into a nightly.
WebVTTParser doesn't do any buffering except for the first line, so if your web server sends a cue in two parts, it will ignore the second half of the cue.
The patch attached to https://bugs.webkit.org/show_bug.cgi?id=81123 might fix this, but it needs to be rebased and tested.