@plungeintIs your game running on android?
I just run TestCpp in version 2.2.1 on android. There isn't any problems with PVR graphics when it going to background/foreground.
There maybe two reasons of the situation you mentioned:
1. The texture is not reloaded when the game going to foreground.
2. The memory of your device is not enough for reloading the textures.
If the assets appear in black color not stable (Sometimes a.pvr becomes black, sometimes b.pvr becomes black), I think the reason is the second one : memory is not enough.
I think you can take a test on many other devices. If the problem always appeared, you'd better upload a demo which can reproduce the problem. Then I can take a look at it, and try to solve the problem.
Thanks for your feedback!