Comma links in LTR texts in RTL arabic document

I insert an english fragment into arabic text. If there is a comma after a digit, I get surprising result. The comma goes before the number, not after.

The font above is "Amiri". I tested also "arabtype.ttf" from Microsoft (http://ift.tt/12cZVia) and got the same result. Therefore I consider it is not a bug, but some important feature.

Question 1, not important, just for curiosity: where can I read about this feature?

A general solution is to somehow remove script=arabic from font for LTR texts. I suspect it is a hard task.

An easy solution for this concrete problem is to add \hbox{} after the number. Then:

The question 2: are there other surprises for english text fragments when using script=arabic?

Sample code:

\documentclass[a4paper]{article}
\usepackage[RTLdocument]{bidi}

\begin{document}

\font\f="Amiri:script=arabic"
\f

\LRE{Print 42, then exit, but not.}

\LRE{Print 42: then exit: but not.}


\end{document}