Should I be concerned if a lymph node swelling behind my left ear persists long after an ear infection clears up?

A little. A lymph node swelling is usually a sign of infection. Generally it goes back down after the infection is resolved. If you have a lymph node behind your ear (posterior auricular node) that is enlarged, it is worth checking out with a doctor. It might be a sign of a residual infection in or near the ear, or could be a sign of a lymph node disorder. Please see your regular general practitioner.
Maybe, maybe not. This is not uncommon. Lymph nodes, once enlarged while helping to fight infection, may not return to normal size for lengthy time periods. On the other hand it never hurts to have an experienced clinician take a look at this and make sure nothing more serious is in progress.