Compiling \begin{align} environment is messed up when you have a skipped line?

Question

I'm learning LaTeX and writing code on overleaf.

This code with a skipped line does not work, it only outputs the second sentence without any formatting ...

\begin{align}
    This is my first sentence.\\
This is my second sentence.    

\end{align}

But this code with a comment placed on the skipped line works and returns the string with math formatting ... why?

\begin{align}
    This is my first sentence. \\
    %
    This is my second sentence.
\end{align}