Melted inline fuse

I had an in-line fuse on the power wire for my electric water pump. It’s a 20amp mini blade fuse that came with the pump and it’s in a weather resistant fuse holder.
Somehow, one of the blade legs burned off, but the actual fuse part of the fuse didn’t blow. How would this even happen?
The wires are in loom and are in perfect shape. I just don’t see why the fuse didn’t blow but the whole thing melted. I cut the fuse out and hotwired it to get home.
My engine got to 260° because of it but seems to be ok
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