That’s odd!…. It’s usually the driver’s side that runs ‘hotter’ due to the heat riser valve, located in the exhaust system directly under the passenger side exhaust manifold, diverting exhaust through the intake manifold and into the driver’s side exhaust manifold.
This would remain true until the valve fully opens and then the exhaust temps basically equalize.
Did someone install the heat riser valve on the wrong side?
Short of that, there must be a restriction of some sort somewhere in the driver’s side of the exhaust system.