Does this happen after the engine is running, or does it fail to start, and the starter keeps running when the key is released? Some starters put enough torque on the drive pinion while cranking that the solenoid spring cannot pull the gear out of engagement with the flywheel until the engine fires and unloads the drive gear. That's particularly true with a new engine that takes a lot of starter torque to turn the crankshaft. Tune the engine properly so it fires up and runs quickly, and that problem will probably disappear. If the starter is continuing to run after the engine starts, you might have a weak or broken solenoid plunger return spring, or a sticky solenoid plunger.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!