You are using a formula/look-up/reference field in Salesforce, the field is synced to Marketo, but you are not seeing updates sync from Salesforce to Marketo.
When a SFDC formula/look-up/reference field is updated, the SysModStamp does not update. The SysModStamp is how the sync knows which record (Marketo vs SFDC) is more up to date. You can think of the formula/look-up/reference field like a window to a different value. If the value inside the window changes, that does not queue the record up to be synced. But if the record is queued up to be synced due to another update made to the record, like changing first name, then the current value inside the window will sync down to Marketo.
To avoid this delay in updating, you may wish to replace the formula or lookup field with a string field that has the appropriate value written to it by a workflow. Updates to SFDC string fields will always update the SysModStamp and queue the sync to Marketo.