ASP.NET website requires login after each compile

I am working on two ASP.NET websites. Both use custom authentication process based on forms authentication with:

<authentication mode="Forms">
  <forms cookieless="UseCookies"/>
</authentication>

set in Web.config.

When I compile the first website, it always remembers my credentials I've entered before, like expected.

When I compile the second website, each time it forgets completely all credentials I've entered a minute before, and .ASPXAUTH cookie is not here nevermore.

What can cause the second website to do so? Where to start to search for the resolution of this proble开发者_高级运维m?

Perhaps when the 2nd site recompiles some class or other object changes that is stored in the session so the application has to restart the session, but in the first site the only thing being stored in the session are standard .net objects that havn't been recompiled. Is this causing a real world problem, it is fairly usual to lose your session state when you recompile a website, but this does not cause any problems i the wild unless you are updating your live code several times a day and kicking users of the system.