I was curious, as I also use Manjaro and Emacs 27.1. I installed JuliaMono
from the git repo. I had trouble getting it to appear in the xfce4 terminal
preferences, but I could see it in Emacs's font picker. Running your code
function and pasting into my *scratch* buffer I see:

[image: emacs-fonts.png]

so, fairly inaccurate placement of the diacritical marks.

Punting on JuliaMono in xfce-terminal I went with Monospace Regular:

[image: fonts.png]

That seems to be pretty bad as well, and Monospace regular should be well
sorted. I know nothing about fonts or diacritical marks, and have a US
keyboard, so sometime ago I stashed a file in my home directory where I
could look up the occasional accented character. I visited that file and
the letters all look fine. Here's a chunk of small letter a code points:

[image: compose.png]

That suggests to me that if there's something wrong it might be with the
width of some of the diacritical marks themselves, perhaps too wide for the
letter they sit above? Still, that theory doesn't explain the wacky
rendering of LATIN SMALL LETTER A WITH CIRCUMFLEX. It also renders fine in
my compose file:

[image: circumflex.png]

I'm just shooting in the dark though, so I could well be off-base. Are you
programmatically inserting characters in your buffer in general, or just
for this example? Does your ā also render badly when inserted via your
keyboard? (FWIW, it looks fine for me using C-x 8 RET LATIN SMALL LETTER A
WITH MACRON RET.)

Skip Montanaro