Contact Owner field in Microsoft Dynamics not showing up in Marketo

We synced a sandbox instance in Marketo with a dev instance in Dynamics. The contact owner email address in Dynamics shows up in Marketo as "Sales Owner Email Address."

We do the exact same sync between Marketo and Dynamics production instances, but the owner email address does not sync up!

Strangely, out of the 1500 records that went from Dynamics to Marketo, the sales owner email address shows up in Marketo for about 20 records only, others are all blank.

Any idea what the reason is? All other fields sync up as desired.