It would also be great if you could first re-test with a more recent
version of Emacs. For instance, 28.1 which was released this year.
Agreed. Unfortunately the environment I'm in is a corporate one. I'll have to build the latest Emacs inside that old version of Linux (Ubuntu 16.04). And that's inside a VM. I can try that but that will take some time (due to some outside constraints).
I'll see what I can do to try in other ways.