How to solve the indentation anomaly of the first item in the problem 1?

Question

I want to fix this indentation issue, like in the picture below (right image is from my MWE).

MWE:

\documentclass[12pt,a4paper]{book}
\usepackage{amsthm}
\usepackage{enumitem}

%%% Ref: https://tex.stackexchange.com/a/613868/293830
\makeatletter
\newtheoremstyle{problem}
  {\topsep}   % ABOVESPACE
  {\topsep}   % BELOWSPACE
  {\upshape\small}  % BODYFONT
  {0pt}       % INDENT (empty value is the same as 0pt)
  {\bfseries\normalsize} % HEADFONT
  {}         % HEADPUNCT
  {1em} % HEADSPACE
  {\thmnumber{#2}.\@ifnotempty{#3}{\hspace{1em}\itshape\thmnote{#3}\newline}} % CUSTOM-HEAD-SPEC
\makeatother

\theoremstyle{problem}
\newtheorem{problem}{}[chapter]
\renewcommand{\theproblem}{\arabic{problem}}

\newlist{problemlist}{enumerate}{1}
\setlist[problemlist,1]{wide,labelindent=1.4em,topsep=6pt,labelsep=1em,itemsep=0pt,parsep=4pt,label=(\alph*)}

\begin{document}

\chapter{Introduction}

\section{Problems}

\begin{problem}
   \begin{problemlist}
      \item The integral equation for a Bloch function in  periodic potential is given region of integration is confined to.
      \item The integral equation for a Bloch function in  periodic potential is given region of integration is confined to.
   \end{problemlist}
   The integral equation for a Bloch function in  periodic potential is given region of integration is confined to.
\end{problem}

\begin{problem}
   The integral equation for a Bloch function in  periodic potential is given region of integration is confined to.
   \begin{problemlist}
      \item The integral equation for a Bloch function in  periodic potential is given region of integration is confined to.
      \item The integral equation for a Bloch function in  periodic potential is given region of integration is confined to.
   \end{problemlist}
   The integral equation for a Bloch function in  periodic potential is given region of integration is confined to.
\end{problem}

\begin{problem}[Optional Title]
   The integral equation for a Bloch function in  periodic potential is given region of integration is confined to.
   \begin{problemlist}
      \item The integral equation for a Bloch function in  periodic potential is given region of integration is confined to.
      \item The integral equation for a Bloch function in  periodic potential is given region of integration is confined to.
   \end{problemlist}
   The integral equation for a Bloch function in  periodic potential is given region of integration is confined to.
\end{problem}

\end{document}