If I have a name which looks misspelled to LibreOffice, I normally mark it with "ignore" and continue editing. Now it is not underlined in red. Sadly the spell check forgets this after closure of the document. So if I open it again the already marked words pop up again as misspelled words.
That is expected, not a bug. Use Add to Dictionary to permanently avoid underline.
If it was as you said, the menu items "ignore and "ignore all" would make no sense. They just fooling the user, who expects that "ignore" means that the word is an exception for the spelling check. What are the "ignore..." items meant for?