Calibre 5.38 / Linux Mint 20.1
There is an inconsistency with the spellchecker UI. If you select multiple words in the list, you can choose "Ignore all selected words" with the right-click dialogue. But the button to the right of the list only allows you to ignore the
final word in the selection.

SUGGESTION: Modify the "Ignore" button command and label so that it applies to
all the selected (highlighted) words in the list.