We never really investigated, but we had found a relation, that it depends on the used XFA version of Acrobat / Adobe Reader. There was a change between A8 / A9.
I am also facing same issue, Acrobat adds Redaction as a Annotation and I have check that after adding Redaction the original content remain same, then why Adobe Invalidating signature after adding Redaction. its seems to me a bug in Adobe Acrobat 9,10 and 11.
Hmm. If earlier versions of Acrobat were doing redaction then leaving the signed document accessible, by doing an append save, then this was a HUGE bug and I'm glad they fixed it. To leave redacted text behind could be a catastrophe. And it seems to me impossible to preserve an earlier signature while removing the text.
Redaction is not an advisory mark up.