When I select text that ends in the middle of the word and add a link, only the selected part is linked. Also, <nowiki/> is added after the link.
For example, in https://en.wikipedia.org/w/index.php?title=Ramat_Yohanan&diff=565789129&oldid=553919950 I selected "South Africa" and added the link. I saw an AbuseFilter warning when I was saving, but saved anyway to report the bug :)
Being a seasoned Wikipedian, I expect the whole word to be linked, although now that I think of it, I can imagine that linking a part of the word can be useful, too. So I'm not completely sure whether it's a bug or a feature.
This may be related to Bug 50127, though the description is not the same.