Hello, I had the exact same problem and it drove me crazy for a while
What finally fixed it for me was going into Font Book on my Mac and checking for any duplicate or weird versions of Helvetica
I found some duplicates and disabled them
Then I ran sudo atsutil databases -remove in Terminal to clear the font cache and restarted my Mac
After that, fonts started showing normally in Opera again

Seems like it’s more of a macOS font issue than an Opera problem
Hope this helps you too