I'm running the most recent version (24.2) of Illustrator CC 2020 on a Mac and am having bizarre font substitution issues.
I opened an old layout with many instances of a font that I no longer have access to. When I opened the file in Illustrator 2020 there was no missing font warning and font substitution did not occur. Instaed of the expected type there was only empty space and a few random characters (mostly numerals). The same thing occurred with several related files, so it wasn't an instance of a single corrupted file.
I tried selecting the text (or rather the lines where the text should have been) and replacing them manually with Myriad Pro. Whole words were missing at the beginning and end of some lines.
I fired up my old Mac which I keep running with older and obselete programs. In Illustrator CS6, the files opened fine: All of the type displayed correctly (in a substitute font) .
I was about to write this forum about the problem and take a few screenshots to document the problem under CC 2020 but now (several hours later) the problem is GONE! I didn't change anything in the Illustrator preferences, move the files or restart the computer.
While I'm happy to be able to see my type again, I'd like to know what might be going on. I'm crossing my fingers that it was caused by stray cosmic rays or something or the sort that's unlikely to happen again, but the episode was a bit worrying.
Do you use font management?
I use font utility for font management with Illustrator. Do you have any auto activate font utility working on either machine? Sometimes I see Illustrator CC taking a bit of time to figure out what the fonts are going to do. You might start by relaunching Illustrator and if you still get unexpected results try clearing the font caches. I do this about once a month because of the amount of font sets I create, activate and deactivate. The cloud based font sets can load slower depending on your connectivity to the interwebs.
Thanks for reaching out. In addition to good information shared above, I'd request checking out this help article as well to know more about fonts & troubleshooting.
Hope it helps.