This is a problem we are aware of. You're correct in saying that there have been two issues with black backgrounds recently. The one that you are experiencing where the chart's background will turn black after some amount of time running, and another whereby the charts background colour would turn black when set to clearColor.
We're having trouble replicating the former issue on our end, and we're still investigating into it. The latter issue has been fixed in v2.6.1, and any user's setting a clear background on their chart will no longer see a black background.
We've had some user's reporting some success by re-setting their chart background colour to its intended colour in 'viewDidAppear', have you tried this at all?
I'll make sure we notify you on any progress with make with this issue. As I'm sure you can understand, bugs that are difficult to reliably replicate can often be very difficult to fix! If possible it would be great if you could confirm whether you are seeing this issue in 2.6.1 too.