Textwidth wrong after wrapfigure

I have included a picture with wrapfigure and everything is fine - the problem is that in my next formula environment, the formula is totally wrong! It seems to me, that the width which I set in the wrapfigure environment also effects the formula. How can I set the width back again to standard?

Here is my code:

\documentclass{book}\usepackage{graphicx}\usepackage{amsthm}\usepackage{wrapfig}\usepackage{mdframed}\newtheoremstyle{mystyle}{}{}{}{}{}{}{0.5em}{}\theoremstyle{mystyle}\newmdtheoremenv{formel}{Formel}\begin{document}\begin{wrapfigure}{l}{8cm}  \includegraphics[scale=0.48]{example-image-a}\end{wrapfigure}text text text text text text text text text text text text text text text texttext text text text text text text text text text text text text text text text\begin{formel}[Formel]$1+1=2$\end{formel}\end{document}

So the text wrapping around the figure is OK, but then width of the formula is wrong. It seems to me, that textwidth is set to the wrapfigure's textwidth, what results in a wrong display.