Fixed it. The pin that retains the hammer had walked out of the right side of the lower (or the factory never installed it correctly to begin with, I'm not sure). I put it back in place and the safety does what it's supposed to. As I said, I'm new to the platform, and I'm not sure how the unsecured hammer negated the safety. So now I have a different concern. If the pin walked from the relatively few rounds I fired, it's clearly a little too loose in the lower. Is there a way to secure the pin? Thanks for all the replies.