Since Full-Flow Staged Engine uses preburned fuel/OX-rich gases in combustion chamber, don't those exhaust fumes produced by the preburner somehow unbalance or 'pollute' the pure combustion of fuel and oxygen in the chamber? I know there is still coming a lot of leftover fuel and OX through the preburner but what about the other gases produced by the combustion?
For instance in SpaceX's Raptor Engine which uses liquid methane and LOX, preburner produces some carbon dioxide and water steam. Don't these gases spoil the pure combustion? Or is the amount of these fumes so insignificantly small?