These words aren’t underlined for my installation with English (UK) but they are if I select everything and choose English (USA) as the language. I guess that the US dictionary might not be as up-to-date as the UK one.
DictionarySpellCheck.odt (34.8 KB)
People volunteer to update dictionaries and maybe there are fewer people in US interested in maintaining LibreOffice.
You could report an enhancement bug, How to Report Bugs in LibreOffice - The Document Foundation Wiki
Or maybe volunteer yourself, Get Involved | LibreOffice - Free and private office suite - Based on OpenOffice - Compatible with Microsoft

