SFDC admins are often suspicious of Marketo in my experience Marketo won't try to update a field in SFDC if there were no changes to that field value. Marketo logs an activity "Sync Person Updates to SFDC" every time it tries to sync any updates into Salesforce, you can see exactly what fields with what values it is trying to update, just click into that activity. If your admins don't want Marketo updating certain fields, it is cleaner to block field updates on the Marketo side. Go to Admin -> Field Management -> Click into the field -> Block field updates. You can block field updates from all sources except for the SFDC sync. This way Marketo will populate the field if it is blank, if it is not blank it will not update it and will not try to overwrite it in SFDC.
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