Since the Mar 31 upgrade of vte to 0.64.0 an underscore (_) on the bottom line of xfce4-terminal 0.8.10 or mate-terminal 1.24.1 is no longer visible. It looks like 1 or 2 pixel lines are missing.
Downgrading vte to 0.62.3 resolves the problem.


Edit Apr 8, 2021: Removed the quote from the GNOME issue tracker here.

I was hoping for a confirmation or denial, but no one came forward.
Maybe my wording in the initial problem report was the cause *), or no one else sees the problem.


*) Therefore I changed the wording of the report.

In the one and only VTE 0.64 issue on https://gitlab.gnome.org/GNOME/vte/-/issues/ the maintainer states that "the line height is taken directly from the font's metrics".

So I reinstalled VTE 0.64.0 and tested whether the DejaVu Sans Mono Book font I use with size 9 is the culprit. And lo and behold: with the sizes 8 and 9 the problems occurs, but with size 10 it does not!
As for me the optimum size is 9 I also tested with Libration Mono Regular, Luxi Mono Regular and Monospace Regular. With none of these the problem occurs and for me the latter is the most acceptable replacement.

The Monospace Regular 9 workaround as mentioned above does not work in an LXDEish environment.
Keeping VTE pinned at 0.62.3 for the time being ...