Most probably the friction caused by a lack of oil helped the cam lobe to dislodge the shim. Most definitely not a problem with your workmanship.
My theory anyways.
This makes it even more suspect. Well from my viewpoint. There is s trick to the installation of those damn things.
I cannot see that cam wear can cause interference to engine rotation. It will only lead to the valves opening less that they should.
That does not sound like a cam wear issue. Sounds more like the cam chain tensioner failed and the chain jumped a few teeth causing interference between valves and pistons.
Kom vir 'n lang vanaksie na die Kaap. Hier kry jy afspraak vir die toets 2 weke vanaf jy dit boek. Dan gaan jy sommer gou Billy toe vir 'n headsup kursus, en voila!
I always start my bike only once I sit on it. And so would many other. It would be interesting to look at behaviour patterns for those customers that reported wear.