I had issues with Copy Files. The solution was to remove it and to use a script to do the coping.
Youlean
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Filed a bug report: FB8969464
I am using the old certificated like on Catalina, so I guess this isn't a problem.
Just installed a 11.0.1 and the bug is still there. So far the workaround is to sign installer on earlier version of macOS and to notarize and do the rest on the Big Sur.
I have tried and notarizing in Big Sur doesn't work in El Capitan or earlier. So, definitely there is some problem with using productsign in Big Sur.