I noticed that when you get to wave 10 it starts upgrading every 10 waves. Maybe it's a bug? Still, it doesnt ruin the fun. Good game!
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Sorry for the late response, but I went back and played up until Wave 24. The game gave me an upgrade every 5 waves; one on Wave 5, one on 10, one on 15, and one on 20 (I died one wave short of 25).
Yeah, I don't see any bugs there (unless there's some oddly specific method that triggers it). Thanks for reaching out, though.