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Firstly, apologies for the delay in responding here and any inconvenience this issue may have caused.
Based on the details shared, looks like the probed endpoint is returning a 404 error, which would prevent the health probe from returning back healthy. Application Gateway expects a 200 response for the default health probe.
You'll either need to define a custom health probe to assume 404 as the expected response as healthy, or change the path of the health probe to an endpoint that would return a 200 response.
Please check this if you need to customize the health probe: https://azure.github.io/application-gateway-kubernetes-ingress/features/probes/
Hope this helps.
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