Thanks Edward - that's the weird part. We have no queue, no lead routing, no auto-assignment whatsoever set up in SFDC other than that default, which I as admin have not changed. We rely on the Marketo flow to create the lead and assign it to the right person, and then create the follow up task.
It used to work where both the newly created lead and the task were assigned to the right person. And now, some change has taken place that I can't track down that SFDC is overwriting Marketo's instructions to default to the admin.
Thanks again - I opened a support case with SFDC in the hopes they can help too. Marketo Support looked at our installation and said it is 100% happening on the SFDC side.