Tapped holes keep stripping in 6061 aluminum

I'm tapping M6 holes in 6061-T6 aluminum for a bracket assembly. Using a spiral point tap and cutting fluid, but several holes strip after only a few fastener insertions. I've tried reducing tap speed, using a pecking technique, and even pre-drilling to the exact minor diameter, but the threads still fail. The holes are blind, about 1.5x diameter deep.

Could the issue be the alloy's softness, or am I missing something in the tapping process? Should I switch to a thread forming tap instead?

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