Ifishsum,
As I explained, the reason would be stripping the lead rather than spinning the bullet properly. He'd have to recover a bullet to check for that, but I don't really see why lacking lubrication would cause it. The same goes for velocity. Why would lack of lube in one shot, firing through a bore that still has lube traces from a previous bullet not shoot the same as the lubed bullets? Very odd that only the unlubed bullets tumble. That would be a reason to recover one.
DukeConnor,
Internet wisdom is often worth what you pay for it. A moment's reflection will reveal the big bullet companies ship boxes of their much softer swaged lead bullets all the time without serious complaints. I got a box of two thousand Remington .358" wadcutters from Midway a year or so ago. The box the bullets were packaged in didn't hold up well against being dropped and had leaked a number of the bullets into the outer box around it when it arrived, but they were all usable.
The reason for hardcast bullets is the broad belief that they may be counted on to produce less lead fouling. It tends to be true if the gun's barrel is straight and lacks constrictions. If it has constrictions, softer bullets will actually do better because the pressure can upset them back up to groove diameter after passing through one, so that the bore remains sealed, where a harder bullet can be loose in the bore after a constriction, allowing gas cutting of the lead which impact plates it against the bore. However, that soft bullet trick fails if the bore isn't smooth.
Soft bullets have an obvious advantage in a hollow-base design because the base blows out to fill the rifling well at moderate target pressures, though it also means you can't produce too much muzzle blast or you will blow that hollow base out into a skirt to produce a lead badminton shuttlecock that has unbelievably awful ballistics. Another advantage to soft bullets mentioned by Hatcher back in 1927 was that they upset easily on impact to result in more stopping power than a same-weight round of hardball has.
An advantage to hardcast bullets is they can be more accurate at higher pressure. The late Richard Lee's book, Modern Reloading, 2nd. Ed. points out that if the peak pressure you are loading to does not exceed the yield point of the bullet alloy, you can see an accuracy improvement on the target because the bullet base isn't being distorted by pressure. You can avoid that much pressure with the following approximation:
Multiply the bullet BHN by 1300 psi/BHN, and don't use loads that exceed the resulting pressure. (The exact number would be 1422, IIRC, but Lee wanted to leave a roughly 10% margin for shot-to-shot pressure variation. The exact number he used corresponded to 1280, but I think 1300 is close enough.)