In the tumbler, with all the rounds hittintg each other, they didn't get swaged down even as little as .001" by chance? Or that little bit of extra lube was actually preventing leading to begin with? It's hard to say for sure. I would more likely believe that the bullets were cast at a different time and the alloy changes just a little bit and they shrunk more than before(closer to pure lead) or maybe harder/softer bhn ect...
I would load up another 50 the same again but leave out tumbling and see if the leading goes awy again( clean the barrel good first). Go from there.
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