Thanks for flagging this. I completely understand how frustrating font inconsistencies can be. Just to check, do you also have DM Sans activated from Adobe Fonts, in addition to the installed version?
Try going to the Creative Cloud desktop app and deactivate DM Sans if it's active there. Then restart Illustrator to see if your system-installed version loads correctly. Also, please confirm your OS version and whether this issue happens only with older files or in new ones too.
It may also help to sign out and back into the Creative Cloud app. If the issue persists, try clearing your font cache using the steps here: Troubleshoot fonts in Illustrator and InDesign
Let me know how it goes or if you're still seeing the diamond icon. A screenshot would also be helpful.
Looking forward to your update.