Very likely, the sockets are not ok. Apparently, the electrical conection of the bulb base to socket has high resistance and is heating causing the plastic base of the lamp to melt.

Carefully examine the connectors in the socket again. Use a magnifier if needed. I suspect a patina has developed on the connector strips. Can you determine the material? It is plated steel and has the palting worn through? Is it brass but not shiny, but has a dull somewhat gray finish? If so, this is high resistance patina. It is bronze and has developed a patina?

Bronze and brasses can develop a high resistance patina and not appear corroded.

who knew sockets cost so much? very hard to see contacts. i need magnifier and spotlight. the contact area is a blade and a small center prong that has a slight bulge. all 4 contacts seem identical while the brake light sockets i saw had different contacts on each end of bulb. but the generic socket is for 4157 bulbs so they must work.

The little dimple you saw is a 'reliability' dimple. It tries to create high pressure on the contact spot.

Use a voltmeter to measure the voltage in the lamp socket to ground. First, look at the wiring diagram and detrmine which prong(s) will be hot with the lights turned on. And be careful that you do not short the connectors together with the meter probe. If you find there is no voltage, start tracing back until you find where you loose it. The problem cound be in the socket, somewhere in the wiring, or the lamp relay. Ons suspect location is the splice where the socket leads are spliced into the body harness.
Inspect all grounds assocatied with that circuit. You'll need a wiring diagram to find all of these. The worst problem to find is a broken wire inside its insulation. This circuit can be intermittent. On GM products, this occurs most often in circuits using aluminum wire, and/or solid wire instead of standed.