Publish Online issue: Video progress/loading not scaled to length of UI progress bar

Sorry for the confusing title! I'm encountering a really weird issue with videos in documents I publish online.

In the screenshot below, you can see that the video has finished, but the loading indicator (and, confusingly, the progress indicator that moves within the loading indicator) on the progress bar is only two-thirds complete. You can see the circle on the right that indicates the position in the video: this moves totally independently of the loading AND progress bars and, in any case, is only visible if you are hovered directly over the bar:

If your cursor is over the video rather than the progress bar itself, here's what you'd see with the video at the halfway mark:

The progress indicator is 50% of the way through the loaded indicator, which doesn't fill the UI progress bar. It's very confusing for recipients, who think the video isn't fully loaded or finished.

If I right click on the video and open its direct link the progress bar is completely normal. The video is a 1080p 24fps mp4 generated from Da Vinci Resolve and then re-encoded with handbrake. Codec details:

 

Has anyone encountered this or got any suggestions? The only thing I can think to do is hide the UI entirely by effectively letterboxing the video with its frame. But I'd really rather not do that.