I suspect this a bug with the version of CR you're using.
I remember answering another question with a similar requirement here and this workaround does seem to work well with export to excel too:
Problems with a single subreport value in an Ex... | SCN
Back then I was on a lower version of CR (I don't remember which one) however, I now have an upgraded version (2013 SP6) and have the same issue as you!
-Abhilash