During tonight’s Falcon 9 launch of Starlink from Space Launch Complex 4 East at Vandenberg Space Force Base in California, the second stage engine did not complete its second burn. As a result, the Starlink satellites were deployed into a lower than intended orbit. SpaceX has made contact with five of the satellites so far and is attempting to have them raise orbit using their ion thrusters.

There's also a tweet saying the same thing in fewer words.

This is the affected mission: Starlink 9-3 launch bulletin

Let's hope it was due to SpaceX pushing the envelope on their in-house Starlink missions in some way, though I have no specific guesses along those lines. Perhaps a manufacturing defect or an operational mistake are more likely to be the leading candidates for the cause.