Discrepancy in vertical spacing when using `\underbrace` in display math

[jfbu]

Description

There is a discrepancy in vertical spacing between xelatex and lualatex output when using \underbrace in display math.

Check/indicate

• [x] Relevant for XeTeX
• [x] Relevant for LuaTeX
• [x] Issue tracker has been searched for similar issues? Yes I found some closed tickets related to overbraces or underbraces (such as #391) but I did not find this issue.
• [x] Links to discussion if appropriate https://tex.stackexchange.com/questions/423078/discrepancy-between-lualatex-and-xelatex-regarding-display-math-vertical-spacing

Minimal example demonstrating the issue

\documentclass[a4paper]{article}
\usepackage[left=0cm,right=12cm]{geometry}
%\usepackage{fontspec}
\usepackage{unicode-math}
\usepackage[grid]{eso-pic}
\begin{document}
Before
\[ \underbrace{AAA}_{B}\]
\[ \underbrace{AAA}_{B}\]
\[ \underbrace{AAA}_{B}\]
\[ \underbrace{AAA}_{B}\]
\[ \underbrace{AAA}_{B}\]
\[ \underbrace{AAA}_{B}\]
\[ \underbrace{AAA}_{B}\]
\[ \underbrace{AAA}_{B}\]
After
\end{document}

Further details

When commenting out unicode-math and loading only fontspec no issue. The issue was observed also when using \setmathfont{TeX Gyre Pagella Math}.

(tested with TeXLive 2018 pretest updated as of today March 25, 2018)

[jfbu]

An edit to the tex.sx question showing the phenomenon in Plain TeX, hence it is appears not related to unicode-math.

Could unicode-math compensate? if that does not make sense, probably better to close this issue ;-)