\pagebreak affects top of next page

Question

Why does a \pagebreak push the heading at the top of the next page down? To match other pages, I need to add \vspace*{-3\baselineskip} after the page break.

\documentclass[a4paper]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{dsfont}
\usepackage{indentfirst}
\usepackage{fullpage}
\usepackage{titling}
\usepackage{fancyhdr}
\usepackage{graphicx}
\usepackage{epstopdf}

\pagestyle{fancy}
\setlength{\headheight}{0pt}
\setlength{\headsep}{32pt}
\setlength{\droptitle}{-4em}
\lhead{-}
\rhead{-}

\renewcommand\thesection{A-\arabic{section}}
\renewcommand\thesubsection{\thesection-\arabic{subsection}}
\renewcommand\thesubsubsection{\thesubsection \ \alph{subsubsection})}

\begin{document}

\subsection{}
\pagebreak
\subsection{}

\end{document}

Answer 1

It is not \pagebreak, but fancyhdr.

If you look at the warnings in the log file, you'll see

Package Fancyhdr Warning: \headheight is too small (0.0pt): 
 Make it at least 12.0pt.
 We now make it that large for the rest of the document.
 This may cause the page layout to be inconsistent, however.

This explains the fact that the second page is moved 12pt down. Comply with the recommendation and you'll not have the problem.