GtkSpell 3.0.4 seems to treat the typographically correct apostrophe as a word boundary. While “doesn't” (single quote) is accepted, in “doesn’t” (apostrophe) the substring “doesn” is marked up as a typo.
Fixed in commit 62d31b5e8130acfcf6890fc805842ed328dcbf7c, note however that the spelling dictionaries typically use the single quote as apostrophe, so doesn’t as a whole word will now be marked as a typo unless the word is added to the custom dictionary (at least, this is the case with the hunspell-en spelling dictionary).
Thanks! Also thanks for the heads-up on hunspell. Aspell seems to have the same problem, sadly, and both still don’t support character equivalence.