fillbetween from pgfplots does not work inside groupplots

This is a bug in pgfplots 1.11. I downgraded to pgfplots 1.10 and everything looks as expected.

The bug has been fixed in pgfplots version 1.13. With 1.13, the example compiles as expected without any changes:

At the time of this writing (January 9, 2016), version 1.13 is quite fresh and might need a couple of days until it arrives in all TeX package managers.