Sounds like a bug, but there have been numerous fixes recently with regards to row indexes. Can you please post a test case that reproduces the problem? I can test against the latest nightly and file a bug if needed. Thanks.
Looks like this is a known issue. This bug seems actually to be the same problem with the rowexpander, and another bug (EXTJSIV-11389) seems to match your description even more closely. The issue is currently scheduled to be fixed as part of 4.2.3.