I believe that missing -iso-level 3 in mkisofs invocation on ppc is the culprit.
A solution tested by a user (including complete restore) is described in https://github.com/rear/rear/issues/2344#issuecomment-601375041 - it is what I proposed in #c2.
Upstream PR merged.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (rear bug fix and enhancement update), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.