Write-backs for MySQL and Filemaker syncing

This is about a Filemaker Server <--> MySQL sync setup.

I have two fields, serialNumber and writeBackNumber, and an altogether separate primaryKey that isn't used for this part.

If I create a new record on Filemaker, everything is fine. the serialNumber is auto-incremented on record creation and synced successfully.

However, If I create a new record in MySQL, I can leave both serialNumber and writeBackNumber blank, but when the sync happens, Filemaker does not auto-increment the serial number and put it in the field. It looks like it's just skipping the auto-enter part altogether and leaving serialNumber blank. This seems like what's going on:

1. Create new record in MySQL. serialNumber and writeBackNumber are blank.
2. Click sync, 1 record gets synced to Filemaker.
3. Record appears in Filemaker with both fields blank.
4. Click sync, that record gets erased from MySQL but is still in Filemaker.

Is this a bug?

Do I need to edit the MirrorSync customization script to perform the auto-enter on hub (Filemaker) pre-sync and then copy that over to the spoke (MySQL) post-sync?