I’ve reported this before, but the pattern seems different in B17, so I thought a new thread might be useful. For some words ending in s, putting a single quote (either a possessive or closing a pair of single quotes) causes the word to be underlined as misspelt.
Examples: include words that I have added to my custom dictionary, such as the name DeVries, but also ordinary dictionary words (such as: theories, stress, fungus). But it doesn’t happen with most plurals (e.g. interpretations, accounts, dreams).
It seems as if the dictionary flags words ending in s unless it identifies them as plurals, but fails to recognise some words (such as theories) as plurals. However, it also identifies proper nouns and allows them to end with a closing single quote, so if you capitalise Theories and then add the closing ', it is no longer mis-spelt.