There a few things to know when it comes to deleting leads/contacts in Microsoft Dynamics.
Marketo doesn’t automatically delete people just because leads were deleted in Dynamics. Rather, a field “Microsoft is Deleted” flag is set to true. You can trigger off this field to delete the record in Marketo if desired.
“Delete Person” flow action: This only deletes a person in Marketo (an option to also delete them in Dynamics is not available).
If a lead is deleted in Marketo (but not in Dynamics) and is updated in Dynamics after that, it would create a new person in Marketo (same email address, new person ID).
If a lead is deleted in Dynamics (but not in Marketo) and then subsequently runs through the “Sync Person to Microsoft” flow action, it would create a new lead in Dynamics.