This is the statement that doesn't make sense. If you put a 12V bulb in a circuit that is going to provide 19.7V, the bulb is not going to survive very long. The fact that it didn't immediately blow is a tribute to the manufacturer of the bulb.

You cannot substitute lower voltage bulbs and expect them to survive more than a few minutes.

Think about the vehicle. If the voltage regulator decides to go AWOL and offer up 17V, all the bulbs in the vehicle shine brightly, but their life is decidedly limited.

Same deal with your 12V bulb.