post-review in RBTools 0.5.2 with --revision-range isn't uploading the parent diff to the RB server. It's generating both diffs but due to a bug in git.py it's not properly registering the parent_diff, causing the caller to ignore it. I've found and fixed the problem; just filing this bug so that others will know about the breakage.
Fixed in rbtools release-0.5.x (50cf71e). Thanks!