TikZ nested text boxes margins

To long for the comment:

• Don't nest TikZ pictures!
• One of possibilities for your node is

\documentclass[tikz, border=2mm]{standalone}
\usetikzlibrary{backgrounds, 
                fit,
                positioning,
                shapes.multipart}
\definecolor{CoreBlue}{HTML}{5b9bd5}
\definecolor{CoreOrange}{HTML}{ec7d2d}
\begin{document}
\tikzset{
    base/.style = {text width=#1, align=center, text=white, outer sep=0pt},
    base/.default = 17mm,
      CB/.style = {base=#1, fill=CoreBlue},
      CO/.style = {base=#1, fill=CoreOrange},
        }
\begin{tikzpicture}[]
  \node (1a) [CB] {Lorem};
  \node (1b) [CO,below=0mm of 1a] {Ipsum}; \\
\scoped[on background layer]
  \node (1)  [CB, fit=(1a) (1b)] {};
\end{tikzpicture}
\begin{tikzpicture}[]
  \node (1a) [CB] {Lorem};
  \node (1b) [CO,below=0mm of 1a] {Lorem Ipsum}; \\
\scoped[on background layer]
  \node (1) [CB, fit=(1a) (1b)] {};
\end{tikzpicture}
\end{document}

or

\documentclass[tikz, border=2mm]{standalone}
\usetikzlibrary{backgrounds, 
                fit,
                positioning,
                shapes.multipart}
\definecolor{CoreBlue}{HTML}{5b9bd5}
\definecolor{CoreOrange}{HTML}{ec7d2d}

\usepackage{lipsum}
\begin{document}
\tikzset{
    base/.style = {text width=#1, align=center, text=white, outer sep=0pt},
    base/.default = 21mm,
      CB/.style = {base=#1, fill=CoreBlue},
      CO/.style = {base=#1, fill=CoreOrange, font=\scriptsize},
        }
\begin{tikzpicture}[]
  \node (1a) [CB] {Lorem};
  \node (1b) [CO,below=0mm of 1a] {Ipsum}; \\
\scoped[on background layer]
  \node (1)  [CB, fit=(1a) (1b)] {};
\end{tikzpicture}
\begin{tikzpicture}[]
  \node (1a) [CB] {Lorem};
  \node (1b) [CO,below=0mm of 1a] {\lipsum[1][1]}; \\
\scoped[on background layer]
  \node (1) [CB, fit=(1a) (1b)] {};
\end{tikzpicture}
\end{document}

I suggest to draw it directly rather than nested node. If they are repeated in your code, put it in a pic.

\documentclass[tikz, border=2mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{fit, positioning, backgrounds}

\begin{document}

\definecolor{CoreBlue}{HTML}{5b9bd5}
\definecolor{CoreOrange}{HTML}{ec7d2d}

\tikzset{
    Core/.style={rectangle, draw, fill=CoreBlue,  draw opacity=0, text=white},
    Core2/.style={rectangle, draw, fill=CoreOrange,  draw opacity=0, text=white},
}

\begin{tikzpicture}
  \def\padwidth{2mm}
  \node[Core2,align=center,text width=15mm] (a) {\scriptsize Ipsum};
  \path ([xshift=-\padwidth]a.west) coordinate (a-l)
    ([xshift=\padwidth]a.east) coordinate (a-r);
  \node[above=0pt of a] (b) {Lorem};
  \pgfkeysgetvalue{/pgf/inner ysep}\wrapnodetbsep
  \path ([yshift=-\wrapnodetbsep]a.south) coordinate (a-b);
  \scoped[on background layer] \node[fit=(a)(b)(a-l)(a-r)(a-b), inner sep=0pt, fill=CoreBlue] {};
\end{tikzpicture}

\end{document}

This is a simple and quick solution/suggestion: use minimum width,minimum height, text width, and align=center. You may make a \newcomnand to fit your need. The above Zarko's answer has some advantage over mine.

\documentclass[tikz,border=2mm]{standalone}
\usepackage{lipsum}
\begin{document}
\definecolor{CoreBlue}{HTML}{5b9bd5}
\definecolor{CoreOrange}{HTML}{ec7d2d}
\begin{tikzpicture}
\path 
(0,0) node[fill=CoreBlue,minimum width=30mm,minimum height=12mm] (A) {}
(A.center)+(90:3mm) node[text=white] {Lorem}
(A.center)+(-90:2mm) node[fill=CoreOrange,text=white,minimum width=25mm,text width=20mm,align=center,font=\scriptsize]{Ipsum};
\end{tikzpicture}
\end{document}