Performance bar is broken for AJAX requests in 11.10.0-rc2

I see this locally, on dev, and on master:

Error getting performance bar results for LdEgVXsp1R4, LdEgVXsp1R4

This is because the response for an AJAX request has two (identical) X-Request-Id headers. I could fix this on the frontend, but I'd rather figure out what broke this on the backend first. Some middleware change?