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imho - a lot has changed under the covers with AIX 7.2. Fortunately, binary compatibility works - which is why I expect the AIX 6.1 ksh still runs.
As you say it runs, then suddenly fails.
Have you rebooted, to see if it fails again - immediately? Or does it work for while, and then fail.
Have you compared ldd /usr/bin/ksh-6.1 with /usr/bin/ksh-7.2?
FYI: I recently tested building programs using AIX 7.1 TL5 and went back to AIX 7.1 TL4. The changes in several libraries, especially the additional ones, broke things (in running ./configure e.g., that had worked for years).
I do not have access to POWER7 and later, so no testing with AIX 7.2.
I do want to point out one more thing. Assume 7200-00-00-0000 is buggy. Update to at least 7200-00-01-XXXX. Maybe that cleans up your issue.