Looks like we can't reproduce the issue or there's a problem in the test case provided.
[ExtJS 4.1.1 ] Broken grid rows background and selection
There is a table
Delete the row with ID 994 and insert above the row with ID 993:
Background and selection is broken. At the same time, grid.getSelectionModel().getCount() == 1
browser: firefox 18.0.1
os: win7 x64
This has already been fixed in a newer release.