The last time I ran some 1018 it had inclusion in the material that stopped the spindle on a A42 Traub screw machine. I have not run 1018 since. If a customer does not to change to 12L14 he can take his work elsewhere.
First thing I thought of when all else is the same. Cut some of that aluminum up and look for chunks of carbide. Others mentioned too soft, harder to diagnose if it is just in spots.
My experience with 6063 is it ran fine as long as you got the coolant to the cut well. As soon as the coolant wasn't quite right, spinweld! With this in mind, I would say try to improve coolant delivery instead of slowing it down.
Supplier probably blew the heat treatment. Aluminum has a very short quench delay during solution annealing (time from oven to quench bath, usually just a few seconds), so it's easy to screw up. Same reason why thick sectioned aluminum bars tend to be more problematic, because the cooling takes longer.
A lot of extrusions are made in China too so there's that.
How much $ do you have invested in the extrusions? A full battery of tensile tests from a lab will run about $100-150. You can also get a few pieces of material heat treated to T6 and pay a lot minimum, which will probably also be around $100-150. Then you can try drilling the properly heat treated material. Not sure what your relationship is like with your material supplier. If we ran into these issues, we would likely get the material swapped and the testing costs comped. As a bonus, the tests would satisfy my curiosity.