We've got a boolean field in Salesforce with correct field mapping, and a flow which seems to be correctly set up, but if an update to the field comes from Marketo activity, it doesn't seem to push through to Salesforce even though the field-level security and accessibility in Salesforce is allowing it. Does anyone have any clue what may be causing this please?
Is the field on both Contact and Lead and mapped properly in both systems?
What do you see in the lead's activity log?
I would check with your SFDC admin to see that you're sync user's profile has permissions to read and write from the field in SFDC.