A strange partial symbol into newtx package

Question

Considering that in these weeks the newtx package is changing versions continuously from my old question Modify characters of mt2pro[lite] using Times, using this MWE,

\documentclass[a4paper,12pt]{article}
\usepackage{newtxtext,newtxmath}
%%%%%%%%%%%%%%%%%%%%%% Nuovo \partial
    \DeclareFontFamily{U}{nxlmi}{}
    \DeclareFontSubstitution{U}{nxlmi}{m}{it}
    \DeclareFontShape{U}{nxlmi}{m}{it}{
      <-6.3>    nxlmi05
      <6.3-8.6> nxlmi07
      <8.6->    nxlmi0
    }{}
    \DeclareFontShape{U}{nxlmi}{b}{it}{
      <-6.3>    nxlbmi05
      <6.3-8.6> nxlbmi07
      <8.6->    nxlbmi0
    }{}
\renewcommand{\partial}{{\text{\usefont{U}{nxlmi}{m}{it}\symbol{64}}\mspace{1mu}}}

\begin{document}
\[\frac{\partial x}{\partial t}, \frac{\partial \overline{E}}{\partial t}\]
\end{document}

I obtained these \partial symbol (number 64 of newtx package).

But compiling with TeXWorks (with last update of MikTeX) I have obtained another symbol, as from image,

The font is pretty but it is small compared to the size of the lowercase and uppercase of each characther. Now also if I remove the command \renewcommand{\partial}{{\text{\usefont{U}{nxlmi}{m}{it}\symbol{64}}\mspace{1mu}}} I will have the \partial symbol of mtpro2 lite.

I have not obtain the \partial symbol that exist into the guide of newtx. Why and what is the strategy to get the partial symbol of the first image?

ADDENDUM 2019/11/1:

As suggested by the very good author of newtx Michael Sharpe that I thank so much, the font nxlmi and its variants contain symbols from Libertine, and are called when I supply the libertine option to newtxmath. It is ntxmi and variants that contain the symbols appropriate to newtxtext. Therefore I have changed my original code,

\documentclass{article}
\usepackage[lite]{mtpro2}
\usepackage{newtxtext}
    \DeclareFontFamily{U}{ntxmi}{}
    \DeclareFontSubstitution{U}{ntxmi}{m}{it}
    \DeclareFontShape{U}{ntxmi}{m}{it}{
      <-6.3>    nxlmi05
      <6.3-8.6> nxlmi07
      <8.6->    nxlmi0
    }{}
    \DeclareFontShape{U}{ntxmi}{b}{it}{
      <-6.3>    ntxbmi05
      <6.3-8.6> ntxbmi07
      <8.6->    ntxbmi0
    }{}
\renewcommand{\partial}{{\text{\usefont{U}{ntxmi}{m}{it}\symbol{64}}\mspace{1mu}}}

\begin{document}
\[
\partial t
\]
\end{document}

But with the new code I get the following error message:

! Undefined control sequence.
\partial ->{\text 
                  {\usefont {U}{ntxmi}{m}{it}\symbol {64}}\mspace {1mu}}
l.23 \partial
              t
? 

If I remove the new definition of \partial -- \renewcommand{\partial}{{\text{\usefont{U}{ntxmi}{m}{it}\symbol{64}}\mspace{1mu}}} -- I get default \partial symbol of the package mtpro2.

\documentclass{article}
\usepackage[lite]{mtpro2}
\usepackage{newtxtext}

    \DeclareFontFamily{U}{ntxmi}{}
    \DeclareFontSubstitution{U}{ntxmi}{m}{it}
    \DeclareFontShape{U}{ntxmi}{m}{it}{
      <-6.3>    nxlmi05
      <6.3-8.6> nxlmi07
      <8.6->    nxlmi0
    }{}
    \DeclareFontShape{U}{ntxmi}{b}{it}{
      <-6.3>    ntxbmi05
      <6.3-8.6> ntxbmi07
      <8.6->    ntxbmi0
    }{}


\begin{document}
\[
\partial t
\]
\end{document}

How can I solve the problem? I am waiting patiently for an possible answer.

Answer 1

(too long for a comment, hence posted as an answer)

There have several updates to the newtxmath package in recent days. In what follows below, I use the version that's current as of today, i.e., 1 November 2019.

First off, here's what I get when I load the mtpro2 package and run $\partial\ \scriptstyle\partial\ \scriptscriptstyle\partial$ in the body of the document:

Next, here's what I get when I load the newtxmath package. The upper row is without the \renewcommand instruction, while the lower row is with the redefinition of \partial. To me, there's not a huge, but still an easy-to-spot visual difference.

\documentclass{article}
%\usepackage[lite]{mtpro2}
\usepackage{amsmath} % for "\text" macro
\usepackage{newtxtext,newtxmath}
    \DeclareFontFamily{U}{ntxmi}{}
    \DeclareFontSubstitution{U}{ntxmi}{m}{it}
    \DeclareFontShape{U}{ntxmi}{m}{it}{
          <-6.3>    nxlmi05
          <6.3-8.6> nxlmi07
          <8.6->    nxlmi0
        }{}
    \DeclareFontShape{U}{ntxmi}{b}{it}{
          <-6.3>    ntxbmi05
          <6.3-8.6> ntxbmi07
          <8.6->    ntxbmi0
        }{}

\begin{document}
$\partial\ \scriptstyle\partial\ \scriptscriptstyle\partial$

\renewcommand{\partial}{\text{\usefont{U}{ntxmi}{m}{it}\symbol{64}}\mspace{1mu}}

$\partial\ \scriptstyle\partial\ \scriptscriptstyle\partial$
\end{document}