Hi,
Before SAP Note 1515670, it was not possible to reverse a reversal document. But after applying this Note either through manual update or through SP update, it was possible to reverse the reversal document. However, now you can maintain your required settings (if reversal should be possible or not by indicating the type of message), by maintaining customizable message F5 905 through transaction OBA5. Hope this helps to understand the system behaviour.
Sridhar