Well, I'm no expert either, but I've had issues.
You don't tell if this is happening within certain projects on your machine, or if it's a problem when you open a project on one machine but not when you open the same project on another machine?
If the latter, I'd re-install archicad, and perhaps update the system by applying the Combo updater from Apple on that machine only. Use a utility like OnyX to clean caches, especially font caches, first. (And backup the machine before that).
If it's the first issue, I'd perhaps just disable the Arial Italic font first. (you must check that Apples Font Book no longer manages fonts, if you've installed FontExplorer). Arial will still look italic when needed I think, but a bit uglier than when using the font.
if needed, backup the font file to some external drive or another network machine, and then delete all copies of it from the trouble machine. Then empty font caches, restart, repair permissions, and check.
After that I guess I'm out of ideas!
AC4.1-AC24SWE-25INT; OSX11.5; MP5,1+MBP16,1