Using IdentityServer4-the login process successfully redirects from VM2 to VM1 when we stop VM2.But, the reverse (from VM1 to VM2) does not happen

Question

My web application is hosted on two Virtual Machine's on Azure and I have application gateway in place on infra level. When I login in application via VM2 and after logged in, I stop the server of VM2. The user is not affected and the request redirects to VM1 and everything works fine. Now when I login via VM1, and after login when I stop the VM1 server, the user gets logged out from application forcefully and it redirects me to the log in page of application from VM2. When I entered the credentials and clicked on login button. The application keeps on loading and I get 401 status code in my api response.

I have implemented Data Protection in my application code, also I am using IdentityServer4 and I have checked the logs where it shows this error:

2023-05-18 09:34:14,587 [138] ERROR [IdentityModel.AspNetCore.OAuth2Introspection.OAuth2IntrospectionHandler] [Error returned from introspection endpoint: Not Found] - Error returned from introspection endpoint: Not Found

 2023-05-18 09:34:14,588 [138] INFO [IdentityModel.AspNetCore.OAuth2Introspection.OAuth2IntrospectionHandler] [BearerIdentityServerAuthenticationIntrospection was not authenticated. Failure message: Error returned from introspection endpoint: Not Found] - BearerIdentityServerAuthenticationIntrospection was not authenticated. Failure message: Error returned from introspection endpoint: Not Found 

What could be the problem? As flow from VM2 to VM1 works fine. But reverse flow is not working.

I have printed the subjectId, email of the user from the claims inside the logs but I got null values for subjectId and email in the logs.