autocomplete after dot broken under certain condition

```fsharp
let foo () =
let a = "a"
(a).Length. "" // cannot complete after the dot
//(a).Length. // if there is nothing after the dot on that line, it can complete
//a.Length. "" // if 'a' is not between parens, it can complete
()
```

I'd expect being able to complete for each of the cases above, but for some reason the first one doesn't propose autocomplete list.

Environment: VS2017, does the same in ionide as well