Why would .ASPXAUTH be ignored?

I've developed a strange problem. I'm using FormsAuthentication to login users. I persist the .ASPXAUTH cookie and set its expiration to today plus 30 days.

The cookie doesn't go anywhere, but the app stops recognizing it. I check it and, indeed, the cookie is still there with an expiration a month away. The only thing "exotic" that I can think of that I do is assign some roles to the principal, but I've done that before without consequence.

Any suggestions?