hi,
it's quite a tricky problem to solve. the situation as i understand it, is:
so jamovi on linux runs inside a flatpak
to perform plots, jamovi uses R
R uses the font settings provided by the flatpak runtime
for a small proportion of users, there's some sort of mismatch between the font settings provided by the flatpak, and what's actually available
so it's the sort of issue that if you approached the R people about, they'd say, go and talk to the font-config people, the font-config people would say to go and talk to the flatpak people, and the flatpak people would say go and talk to the R people.
raising the level of complexity are two additional things: 1. i can't reproduce it on any machine i have access to 2. i know nothing about font-config

so at the moment i'm not feeling optimistic.
sooner or later we'll get our ducks in a row, but it's the sort of issue where you have to wait for some sort of breakthrough somewhere.
if it's any consolation, i'm atleast as frustrated as you are.
cheers
jonathon