ELB HTTPCode_Target_5XX_Count is zero even when I'm seeing lots of 502s.

I have an elb with a target group using the dynamic port mapping ECS mechanism. When I trigger a 4XX from the target, say by asking for a route that doesn't exist, I can see the HTTPCode_Target_4XX_Count change in cloudwatch; however the corresponding 5XX metric does not change when I trigger any 5XX responses from the target. I can see that the status code is set correctly (Chrome thinks it is a 5XX request), but the HTTPCode_Target_5XX_Count remains '-' indicating no data. I've tried setting the statistic to sample count and the period to a minute. I've also tried setting the statistic to Sum as indicated in the docs, but no combination of statistic/ period gives any data - even though I'm receiving 5XX errors from my target (routed through the ELB).

I've tried in two different vpcs with the same results.

Thanks!