Your software
My Mautic version is: 2.16.2
My PHP version is: 7.3.17
Your problem
My problem is: When using the Salesforce integration, the Mautic Contact ID does not sync to the corresponding Salesforce Lead record. In the plugin Contact Mapping, the direction selected appears to be correct but greyed out (as expected). However, no data appears in Salesforce.
Salesforce data transferring across to Mautic is working fine. Am I missing something simple? 
These errors are showing in the log:
None
Steps I have tried to fix the problem:
Re-run app/console mautic:integration:synccontacts --integration=Salesforce