Which version of F# Data are you using? And how are you referencing the type provider?
I just tried the following using the latest version of F# Data (built from source) and this works fine - however, the latest version fixes a number of bugs so it is quite possible that this was not working earlier. Can you try this using the version FSharp.Data 2.0.0-alpha6 (available on NuGet):
#r "System.Xml.Linq.dll"
#r @"C:\Tomas\Public\FSharp.Data\bin\FSharp.Data.dll"
open FSharp.Data
type X = XmlProvider<"""<root>
<Points>
<Point X="0.0" Y="2.0"/>
<Point X="159.3546" Y="3.0"/>
</Points>
</root>""">
let x = X.GetSample()
x.Points.GetPoints().[1].X