Looks like that crank was not heat treated properly. Too much crystallization, looks more like sintered bronze than cast. Good Luck getting the customer any warranty, because no tech will believe that you can break a crank on a string trimmer.
Well might not believe you but the proof is in the pudding. I view point about things is anything is possible, even what might seem impossible is possible, In this case you are probably dealing some that high quality Chinese cast metal that has a case of Covid...
It might be like the time I had 100 calcuators adding wrong. Victor didn't believe me; until, they pull a new off the assembly line and ran my numbers through it. Boy changing over 100 cpu chips was not fun.
Little more information. The bolt that holds the flywheel pulley on the crankshaft is missing, and doesn't appear to be even installed from the factory. The other side of that pulley has no evidence of the bolt or washer being loose. The belt pulley rides above the axle in the picture so would of rattled around until it did what you see. Have started warranty authorization with MTD and waiting on warranty drop ship of engine.
And @Rivets you are right that crank looks like garbage. My assessment is the bolt was missing from the factory. The trimmer has only been run a couple of times. And since the belt pulley rides above the axle it would stay on the crank shaking around until is stress cracked the crank to the point of failure.
Can’t remember how long ago, but Kohler had a problem with crankshafts not being hardened properly, breaking with less than 50 hours of service. I do remember having to work with tech rep and replaced so many that I could do a replacement in less than one hour. He and I got to be good friend even got a couple of nice meals and drinks on Kohler when we put in a couple of 12 hour days.