find the solution!
your also need to delete incorrect word from:

/System/Volumes/Data/Users/username/Library/Spelling/LocalDictionary

and reboot your mac.
after that - everything is ok!

@bo3gyx I tried to delete that incorrect word from the file, tried to delete this file, tried to add new word into this custom dictionary but opera thinks that the first incorrect word is still correct.
Also, opera restart and reboot of mac not helped...

Hi! Same problem here.
I find out, that custom dictionary is stored at: /System/Volumes/Data/Users/username/Library/Application Support/com.operasoftware.Opera/Custom Dictionary.txt