None of the web app health metrics report any data. Have tried Meraki support, and they came back with:
I spent some time on this case today and observed that your MX is currently in passthrough mode, which means the security appliance is acting as a Layer 2 bridge and does not route or translate client traffic
Only traffic that passes through an MX can be tracked by Meraki Insight. For example, if the MX is handling all routing for a location then both WAN bound and Inter-VLAN bound application traffic will be tracked. However, if Inter-VLAN routing is happening on a downstream device and only WAN bound traffic passes through the MX then only WAN bound application traffic will be tracked.
I'm a bit confused because all traffic goes through the MX and Meraki don't state in the documentation that Insight web app health does not work in passthrough mode.