By default \maketitle inserts a new page (via \newpage) before your actual metadata (like title, author and date). This isn’t always the desired behavior (e.g. when you try to put some text before your title). Fortunately there is a way to circumvent / avoid the explicit pagebreak:
{\let\newpage\relax\maketitle}
Switching from TCPDF to dompdf – due to image rendering / scaling / quality difficulties with TCPDF – I (unsurprisingly) stumbled upon some differences how dompdf handles the provided HTML. One of which is the way explicit page breaks can be accomplished. TCPDF expects you to use a special pagebreak attribute:
Whereas dompdf pays attention to the page-break-* CSS properties. A TCPDF-style replacement for an explicit page break might look like this: