Video/Slide will not load in Chrome (PC) if Closed Captions are active

Hi there. I have a module with three media slides playing short videos interspaced between some information slides. The module uses a bespoke navigation system and media Play/Pause buttons, etc (basically not relying on the built-in Player) and so there is also a dedicated button to toggle the Closed Captions on and off.

The module has master slide coding that allows the CC to automatically appear on subsequent media slides if the user toggled them on on an earlier slide. This has worked smoothly for me for a long time on loads of projects.

Now, all of a sudden, in Chrome only, once the user has activated the Player.DisplayCaptions on an earlier slide with a video, and they then progress through to another slide with video, that slide always fails to load (spinning loading wheel for eternity). Quitting then resuming where left off loads the slide, fine, until the next media/video slide is reached.

This only happens when Player.DisplayCaptions is active. When it is false the video slides load and play as expected.

Any idea why this is happening?

Amazingly, this only happens in Chrome (PC Version 70.0.3538.77 (Official Build) (64-bit)) - IE11 works fine - go figure!