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.