from: https://learn.microsoft.com/en-us/azure/frontdoor/health-probes
Complete health probe failure If health probes fail for every origin in an origin group, then Front Door considers all origins unhealthy and routes traffic in a round robin distribution across all of them. Once an origin returns to a healthy state, Front Door resumes the normal load-balancing algorithm.
enable diagnostic settings and search for failing probes, note that successful probes will not be logged:
AzureDiagnostics
| where Category == "FrontDoorHealthProbeLog"
| distinct result_s
set origin group latency sensitivity to 0 to make sure requests are always routed to the fastest origin and skip load balancing.