The format generated by System.Web.HttpCookie when formated is non-standard, a mix between RFC1123 and RFC850 (At least as of .NET Framework 4.6.1)
The exact format string used is
"ddd, dd-MMM-yyyy HH':'mm':'ss 'GMT'"
This doesn't quite match RFC1123 which is
"ddd, d MMM yyyy H:m:s 'GMT'"
as it uses dashes between the day, month, and year instead of spaces, but doesn't quite match RFC850 which is
"dddd, d'-'MMM'-'yy H:m:s 'GMT'"
as it uses a short day of the week instead of a longer form.
The CookieHeaderValue.TryParse ultimately calls the internal
which uses a list of allowable date formats, however the one used by HttpCookie listed above isn't one of them.
Also according to
If the attribute-value failed to parse as a cookie date, ignore the cookie-av.
Meaning, If for any reason the cookie fails to parse the date, it should just ignore this value, however the code fails to parse the entire cookie instead.