OK, so I have an answer, and it's stupid, but I'm going to leave the question and post an answer here in case it helps someone. In this case, the max queue size was set to 1, so the system was dropping the other span. That value is an erroneous and historical wart that I hadn't noticed until I'd put way too much work into debugging this.

Long story short: if you're seeing this behavior, check your configuration for dumb values. :)