I was hit by this problem too but in a different flavor. My problem was not that the sgid bit was not propagated but that we were pushing into a directory with the sgid bit set on the server and that this sgid bit was erased with bzr executing chmod to set the group bits. Since I am using rich-root-pack — which implies as a side effect that no directory will be created after the initial push —, a workaround is to set again the sgid bit by hand...
I was hit by this problem too but in a different flavor. My problem was not that the sgid bit was not propagated but that we were pushing into a directory with the sgid bit set on the server and that this sgid bit was erased with bzr executing chmod to set the group bits. Since I am using rich-root-pack — which implies as a side effect that no directory will be created after the initial push —, a workaround is to set again the sgid bit by hand...