I started to experience this with installers made using PackageMaker and Packages and found the solution was to move from using RTF files over to plain text files for the installer's UI license and readme text sources. I used RTF previously to allow a little light formatting in the license shown to users, etc. Notarisation worked fully before signing the pkg, but after signing they would fail as described with a claim of a corrupt archive. Very peculiar problem needing baby-steps forwards to work out what the problematic delta from a basic working sample was, although this solution makes very little logical sense to me I figure it would be good to share what I did in case it helps somebody else.

Yes. They're pretty simple files made in TextEdit. Checking against this list:https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/AttributedStrings/Tasks/RTFAndAttrStrings.htmlThe first line in one of the files is:{\rtf1\ansi\ansicpg1252\cocoartf1504\cocoasubrtf810I checked again and one of the files that had started failing to notarise has started working again, so I'm guessing something has been changed server side to resolve an issue here. Thanks.

I found some installers not working still. Checked the xattr and yes, various, including some that had com.apple.quarantine which I guess could be causing a problem.

I pulled the extended attributes off the RTF files and it resolved it for one of the ones I was still failing to get through. Thanks.