Please upgrade FSharp.Compiler.Service to the latest version
It looks like the version of FSharp.Compiler.Service LINQPad use is really old - if I'm not mistaken it is 1.4.2.1 which was released several years ago (19 Oct 2015).
This explains lacking IntelliSense new additional.
This explains lacking IntelliSense new additional.
Comments
http://www.linqpad.net/download.aspx#beta
Let me know how you get along.