I did a quick search of the forums and didn’t see this bug reported. Sorry if it’s a repost.
I’ve just downloaded 2.9.0.3 beta, 64 bit version, and am running it on Windows 10.0.16299 Enterprise.
When using the “related word” feature on a highlighted word, instead of replacing the word with the selected word, Scrivener inserts it into the word, wherever the cursor is. If the word is not highlighted, the feature works as intended, by replacing the whole word with the selected replacement.