I agree that spindles shouldn't fail, but, though rare spindle failures aren't unheard of (all and any spindles).

Chalk it up to metal fatigue. Combine small variations with everyday wear and tear, and flexing under load and all spindles will failure eventually. With good design and construction eventually will be a long time coming, but some percentage will fail sooner.

How many fail is a statistical consideration, but you have to accept that zero failure is impossible, so designers try to fine a strength (and weight) where early failures will be acceptably low in number. At some point the added weight needed to push the failure rate lower becomes disproportionate to the improvement, so that's where good design will leave it.

So, I wish I had better to tell you, other than that you drew the short straw. But some risk of mechanical failure, including catastrophic failure of critical parts goes with the territory.