Seekbar empties when triggering a layer

Hi everyone, 

I'm building a course with a seekbar, which works absolutely fine until I get to one slide where it suddenly empties when I click the button, triggering a layer. Then it stays empty for the rest of that slide, then refills when you move to the next slide. 

I've checked the properties of the layer and tried the various options, but it doesn't seem to make a difference. 

Here is a video of it happening, unfortunately it's a bit long so feel free to jump through it. https://360.articulate.com/review/content/c8c4146f-ce25-4a86-8658-d6c2c625ac69/review

Any help on this would be greatly appreciated.