Enumitem identation issue

Question

\documentclass[10pt, a4paper]{article}
\usepackage{lipsum}
\usepackage{enumitem}
\setlist{topsep=1cm}

\begin{document}

\lipsum[66]

\begin{itemize}
\item[\textbf{(P1)}:]
\end{itemize}

\end{document}

The enumitem setting makes the item lose its indentation. Why? I tried this with no success:

\documentclass[10pt, a4paper]{article}
\usepackage{lipsum}
\usepackage{enumitem}
\setlist{topsep=1cm}
\setlist{labelindent=\parindent, leftmargin=*}


\begin{document}

\lipsum[66]

\begin{itemize}
\item[\textbf{(P1)}:]
\end{itemize}

\end{document}

Answer 1

The problem you're having has nothing do do with enumitem and everything to with the fact that you're trying to fit a large label into a small space, so the label appears to change the indent but in reality it is simply expanding to the right. Presumably you expected the label to start left aligned at the indentation but that's not how list labels are usually set.

Here's your minimal example without loading enumitem and you can see that the effect is identical:

\documentclass[10pt, a4paper]{article}
\usepackage{lipsum}

\begin{document}
\lipsum[66]
\begin{itemize}
\item some text
\item[\textbf{(P1)}:] Some text
\item Some text
\end{itemize}

\end{document}

So the solution to your problem (which is easily solved by using enumitem) is to adjust the margins of the list appropriately so that you have enough room for the larger label text and you have an indent of the items. This requires changing both the leftmargin and the labelindent parameters.

Additionally, since you are misusing the itemize list to form what looks like a numbered list, you should really format the label of the list using enumerate instead of itemize. And in general, it's almost always a bad idea to supply an explicit label to a list item, and you should avoid doing it wherever possible.

\documentclass[10pt, a4paper]{article}
\usepackage{lipsum}
\usepackage{enumitem}

\begin{document}
\lipsum[66]

\begin{enumerate}[label={\bfseries (P\arabic*):},leftmargin=3\parindent,labelindent=1.5\parindent]
\item some text
\item \lipsum[66]
\end{enumerate}

\end{document}

Answer 2

After your inputs and further investigations I managed to get what I wanted with the following code (option wide was the key to the desired indentation):

\documentclass[10pt, a4paper]{article}

\usepackage[french]{babel}
\frenchsetup{StandardLayout=true}

\usepackage{lipsum}

\usepackage{enumitem}
\setlist{topsep=1cm, wide, itemsep=0pt, leftmargin=3\parindent, rightmargin=3\parindent} 


\begin{document}

\lipsum[66]

\begin{enumerate}[label={\bfseries (P\arabic*)}:]
\item
\item
\item
\end{enumerate}

\lipsum[66]

\end{document}