ZCS 5.18 to 6.0 upgrade - mail missing

[mtapman]

***Correction/Update: 5.0.19 is the older server version ***

Scenario:
Multi-server 5.18 installation. New 6.0 single server installation. Using the zmztozmig tool to perform the actual mail migration.

Actual results:
New server up and running and configured without a problem. Executed zmztozmig to transfer account data and everything looked OK except some recent mail was missing (about 10 days worth). ztozmiglogs show no errors, response message is "OK".

Any thoughts on why the recent mail would be missing? Single mail store setup, not using any HSM settings. Mailboxes are about 500MB - 1.1GB.

Looking at options for working around this. One seems to be moving to a backup/restore transfer from the old 5.18 install (multi-server) to a new single server 5.18 install, then performing the 5.18 to 6.0 upgrade on that server once the mail is transferred over correctly. However, I'd prefer to figure out the problem rather than work around it.