This happens with all documents in LibreOffice. If a word is not in the dictionary, usually because it is a technical word or brand name, etc, it highlights it as a spelling error. When I right-click on the word and select Ignore, it goes away. Until I edit the document in ANY way, or close/re-open it, and it re-appears again. Am I not understanding how this works? Polluting my dictionary by selecting ‘Add to dictionary’ is not an acceptable option.
I’m using LibreOffice 188.8.131.52 on Arch Linux.