I’m flummoxed why you lot are going on about the bearing preload nut.
It serves no retention purpose. All the work is done by the pinch bolts.
This happened to me once on a new set of ht2 cranks. I installed the bolts without grease and they weren’t cranked as hard as I thought. I had to relctantly conclude user error, even though I’d built loads of bikes, I made a mistake.
The spline in the crank arm was damaged and it never fitted properly tight again and persistently worked itself loose so I had to bin it after that.
You’re a big lad ton, you’ve just dragged it off the end of the axle.