Hello,
To override this default behavior and allow synchronization of such contacts that do not have a parent customer, you need to add this subscriber in an extension and publish it to the environment (tenant):
[EventSubscriber(ObjectType::Codeunit, Codeunit::"CRM Synch. Helper", 'OnGetIsContactBusinessRelationOptional', '', false, false)]
local procedure HandleOnGetIsContactBusinessRelationOptional(var Optional: Boolean)
begin
Optional := true;
end;