I think BadAim is right. My grandfather was a bofors AA gunner during WW2 so I've got some books etc on the subject, found this in one of them.
The Bofors used a standard high-explosive shell with a sensitive point-detonating fuze designed to burst upon impact.* If the shell did not make contact with a target, the fuze destroyed the projectile at the tracer burnout range of approximately 3,500 yards.
I have no idea how reliable this is but it seems to make sense.
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