Why \protect doesn't work with \linebreak?

Question

This question is merely educational and it does not intent to find a solution to a problem, please don't provide an answer like "just use \\ instead of \linebreak".

In the following code, the compiler returns an error Argument of \reserved@a has an extra }., which I guess is because \linebreak is fragile, and somehow using the titling package triggers it.

\documentclass{IEEEtran}
\usepackage{titling}
\title{My Custom Title}
\author{Author 1,\linebreak
Author 2}

\begin{document}
\maketitle
\end{document}

The output of the code without using the titling package is

When adding the \protect keyword the output remains the same. However when adding the \protect keyword and using the package \titling, the output is

Note

The \linebreak could be replaced by \\ which does not seem to be fragile, or at least, it doesn't give the error mentioned above.

Answer 1

The titling package follows the standard classes article, report etc and implements \maketitle using a tabular to set the author, so \\ here is the end of row in a tabular and \linebreak does nothing in a c column, so it is not really about robustness, and \protect therefore has no effect.