I'm getting a LOT of value out of the Heat Streams custom visual!
I have run into what appears to be a layout bug when
1 - a background is enabled on the visual and
2 - the visual's height exceeds some threshold
When this threshold is exceeded appear to render OK, but the associated category values towards the bottom appear to be rendered *behind* the visual background. In the case of a black backround, black category values will be invisible. In the screenshots below, I have adjusted background transparency to demostrate the effect, though in my working example, transparency is set to 0%, making the text totally invisible.
For now, I'm working around the issue by disabling the background, but this doesn't look correct in the context of the rest of my report pages.
It seems Heat Streams always use white background. We'll report this issue to developer to consider in new versions.
Microsoft Power BI Custom Visuals