Why does setting \russianfont cause fontspec to find no support for Cyrillic in a font which works with \setmainfont and Russian as the main language?

Question

This example is from ScumCoder's question, based on Grigory M et al.'s answer.

If I compile the following code with XeLaTeX, all is well:

\documentclass{article}
\usepackage{fontspec}
\usepackage{polyglossia}
\setmainlanguage{russian}
\setotherlanguage{english}
\setmainfont{Linux Libertine O}
\begin{document}
Привет
\begin{english}
Hello!
\end{english}
\end{document}

However, if I add a definition for \russianfont, I have problems:

\documentclass{article}
\usepackage{fontspec}
\usepackage{polyglossia}
\setmainlanguage{russian}
\setotherlanguage{english}
\newfontfamily\russianfont[Script=Cyrillic]{Linux Libertine}
\setmainfont{Linux Libertine O}
\begin{document}
Привет
\begin{english}
Hello!
\end{english}
\end{document}

even though I still have \setmainfont{...} which should set the font for Russian.

In this case, XeLaTeX claims that the font does not support Cyrillic. The same happens if I switch Russian and English so that Russian is an other language (so defining \russianfont might really be needed).

\documentclass{article}
\usepackage{fontspec}
\usepackage{polyglossia}
\setmainlanguage{english}
\setotherlanguage{russian}
\newfontfamily\russianfont[Script=Cyrillic]{Linux Libertine}
\begin{document}
\begin{russian}
Привет
\end{russian}
\begin{english}
Hello!
\end{english}
\end{document}

I am not certain how to tell which copy of the font XeTeX tries to use. I have this installed for both the OS and as part of TeX Live. Moreover fontconfig knows about the TeX Live directories.

fc-match -v "Linux Libertine O" returns /usr/share/fonts/otf-linux-libertine-ibx/LinLibertine_R.otf (and a bunch of information).

However, the fonts installed for TeX Live and those installed for the system appear to be identical in the case of Linux Libertine. diff -qr run on the two directories finds no differences.

What is going on and how should I be doing this?