I uploaded leads from a CSV into an event in Marketo. It says that the leads were uploaded successfully to marketo and I can see them. However, in Salesforce I do not see the full number of leads but only about 10% of the number of leads in Marketo.
Do you have a global sync campaign to SFDC? If so, you could check the results of that campaign and see if something is breaking with the sync. I suspect you might have required fields in SFDC that aren't populated in your list.
Leads created or imported into Marketo remain "Marketo-only" until there is an explicit flow action to sync to Salesforce or if a program is synchronised a a Salesforce campaign which will synchronise all members of that program.