Creative, it had to happen eventually.

I don't know about interchangeability of the closure pins but wouldn't be surprised that they're not. Shimano makes a number of pins for their chains with different distances between the flares on the pins, so it's likely that the pin is too narrow (short) for your chain.

Don't take this personally, but on my wall is a sign that reads "Nothing is foolproof, because fools are too ingenious". I think you might have proved this.