JonSagara-6725, Thanks for the question. Apologies for any inconvenience with this issue.
By default, a client is bound to a specific instance for the life of their web session, by ARR Affinity.
If maintaining session affinity/stateful is a requirement for your WebApp, you may enable ARR affinity setting via Azure Portal.
As for monitor App Service instances, you may use Health check feature - which helps to monitor App Service instances. With the Health check- it increases your application's availability by rerouting requests away from unhealthy instances, and replacing instances if they remain unhealthy.
In a nutshell, the App Service Plan (ASP) is the scale unit of the App Service Apps. If the plan is configured to run 4 VM instances, then all apps in the plan run on all 4 instances.
--If the plan is configured for autoscaling, then all apps in the plan are scaled out together based on the autoscale settings. You pay for the ASP, not for #number Apps.
Always On is not turned on (default), the app is unloaded after 20 minutes without any incoming requests. With Always On - Keeps the app loaded even when there's no traffic.
Kindly let us know, I'll follow-up with you further.