Siemens Panel Main Circuit Breaker Teardown
Send the breaker back (as a warranty return), get some main lugs instead

This breaker is clearly NFG, and thus needs to be sent back under Siemens' warranty (it's a 1-year warranty, but that should do the trick for an infant mortal like this one).  While you're at it, what you want for a subpanel sitting right next to the main isn't a main breaker, but a set of main lugs instead -- the correct part for your panel will be a Siemens ECMLK125 kit.

This is because the busbars on such a panel are an extension of the feeder, so they are protected by the feeder breaker as the feeder tap rules aren't in play here, and the upstream and downstream panels are in the same building, so the downstream panel does not need to provide a master shutoff switch for power to its circuits.