Issue
When using the "Sync to Salesforce" flow step and setting up choices, selecting "Do Nothing" uses the Auto-Assignment Rules in Salesforce.
Solution
This is expected. For this flow step, the choices are basically asking whether the record should be assigned to a specific person or not. When it is not specified (i.e., "Do nothing") the assumption is that you still want the record to be synced to Salesforce, otherwise they would not be in that step, but that there's no particular desire to assign it to a specific person. Thus, since SFDC records need someone to be the owner, the Auto-Assign rules are used by default
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