As the title says, libreoffice doesn’t correct words with apostrophes in them, so possessive names in the middle of a sentence remain lowercase, even after setting the word to autocorrect. It capitalises fine when at the front of the sentence, but if the word is fictitious, and you set it autocorrect, it capitalises but doesn’t correct the error.
On Arch Linux 5.17.5-arch-1, this happens in both libreoffice-still(7.2.6.2) and libreoffice-fresh(7.3.2.2) with the en_ZA(English South Africa) language pack.
Edit:
Example:
Type out:
walked over to hilma's car
expecting:
walked over to Hilma's car
after pressing space after the name. Even after selecting “Always Correct to” option, it won’t change when typing that name again.