Hi Vivian,
It sounds like most likely your Marketo sync user in SFDC doesn't have permission to edit the value of that field. If it's field-level security is set to read-only, then Marketo won't be able to edit the value and will change it back to null to keep it consistent with your CRM.
Additionally, you may need to customize the picklist of that field in SFDC to allow the value. Go to Setup > Customize > Leads > Fields, and click the name of the field. From there, you can both set which profiles have write permission (click "Set Field-Level Security") and edit the picklist values.
If neither of those is the problem, try logging into SFDC using the sync user's credentials and edit the record. If you CAN edit that field, you may want to reach out to Marketo Support (be sure to attach screenshots where you edit the field as the sync user in your support case). If you can NOT edit the field, you may want to reach out to SFDC support for troubleshooting that user's permissions.
Best of luck!
Grant