Have you tried simply restarting the Mac? If that doesn't clear it up...
Due to the way Microsoft & Apple are doing things "up to date" doesn't mean very much :-) Please provide the specific Version of Office as indicated in Word> About Microsoft Word as well as indicating the exact version of macOS as indicated in **> About This Mac**. Causes & cures can vary based on those specifications.
Additionally, do you have any add-ins [such as Grammarly]?
Some alternative selection techniques to consider:
- Click at the start, scroll if necessary, then hold Shift while you click at the end of the passage
- To select a word double-click the word
- To select an entire sentence beginning to end hold the Command key while clicking anywhere in the sentence.
- To select an entire paragraph triple-click anywhere within it
All of the above can be extended on the same basis by using the Shift-Click method. E.g., Command-click a sentence then Shift-click in another to include it as well as all sentences in between.
The left margin is a selection area as well:
- Click to the left of a line to select the line
- Double-click to select the adjacent paragraph
- Triple-click or Command-click to Select All
- Drag down (or up) in the margin to select line-by-line
Unfortunately, dragging is the first & only method most users pick up on their own, so that's what they're most comfortable with. Introducing other methods may seem awkward at first but soon become second nature.