How do I sync contacts that are created in Dynamics 365 to Marketo?
You mean with the native integration turned on? And no sync filter? You don't have to do anything as all leads and contacts will be created in Marketo after initiating the sync.
The integration is turned on however only leads sync back to Marketo, not contacts that are created in Dynamics.
Well, that's not an expected behavior if you don't have any custom sync setup in place to filter out the contact records from syncing over to Marketo from Dynamics. Could you check if you don't have a custom filter setup? There could be an automated workflow setup in MSD that might be updating the custom sync filter so that the contact records do not sync over to the Marketo. You could also double-check if the sync user in Dynamics has sufficient permissions to the contact object.