Common pc mistakes:
Alloy doesn't matter when it comes to leading. You can use pure/dead soft lead and 50,000+psi loads in a 308w with bullets that were cast out of pure lead and pc'd & have no leading in 22"/25"/26"/30" bbl's.
Under-cured pc will do exactly what you described. Typically you can tell by the color of the finished product. The finished coat will be either dull or lighter in color when it's under-cured. Using clear pc was clearly not a good choice.
Using the wrong expander will cause leading also. This show up with under-cured coating on the bullet and swaged down bullet bases. Any scraping of the lead from bullets not being either started strait or the cases not being belled enough will cause leading.
The smash test only tells you so much. Bullets with under-cured pc will pass the smash test. But when hit with high pressure loads and rifling cutting into it, they will lead the bbl's.
Several years ago I decided to start pc'ing, I saw the "smash" tests being done & that got the wheels turning. Specifically 30cal rifle bullets. A picture of 2 bullets I pc's, left is a 301" 212gr paper patch bullet & right is the lee 230gr blackout bullet.
As you can see the bullets were pc's and ran thru a swaging die that turned them into a completely different design. Recovered bullets from the 100yd berm, was testing alloys for hunting bullets. That 2300fps bullet is a lee 230gr bl cast bullet that was loaded with a 50,000+psi load.
The pc/alloy combo's tested above showed 2 things. It took the bullet to hit something hard for the pc to come off. And that that 230gr bullets alloy is an excellent alloy for a hunting bullet. The others shattered & the 2300fps bullet bent.
More cast 30cal bullets (lee 312-160-tl) to test in the 308w. Traditional cast/lubed vs pc'd.
Typical test target for those bullets pictured above in the 308w being pushed to 2600fps+.
When the pc coating is applied correctly there is no leading.