@ElanR1 This was escalated in support, it appears to be an issue with the Marketo - Update a record connector, not mapping fields for Record Type = Leads only.
I do thank you for your thoughts and feedback... ultimately the 105.id is fetching a Marketo ID.
I previously tried removing the 105.id entirely and used a static Lead ID from Marketo, but the issue persisted.
Also, I did recheck the ins and outs of the 105.id (which is a Marketo - Search records connector), and no errors of any sort.
Adobe Support is looking into it - there might be a new bug in the Marketo - Update a record connector. Fingers crossed for an expeditious fix.
Again thank you for taking the time and brainstorming with me here.