Indeed, I retested using the following example file:
$ realpath example.tex
/home/werner/test/example.tex
$ cat example.tex
\documentclass{article}
\begin{document}
The well known Pythagorean theorem \(x^2 + y^2 = z^2\) was
proved to be invalid for other exponents.
Meaning the next equation has no integer solutions:
\[ x^n + y^n = z^n \]
\end{document}
The results show that all the methods discussed here are vaild:
NotebookPut@Import["/home/werner/test/example.tex", "TeX"]
or
NotebookPut@Import["/home/werner/test/example.tex", "LaTeX"]
or
A = Import["/home/werner/test/example.tex", "TeX"]
NotebookPut[A]
or
A = Import["/home/werner/test/example.tex", "LaTeX"]
NotebookPut[A]
There must be something offending in your file MS.tex. If it is really important to you, you may try to track down the problem by removing parts of the TeX file and see what happens.