A family of Microsoft word processing software products for creating web, email, and print documents.
Had this issue myself. Found a fix.
To fix this: Hit the File tab, go down to Options, and in the General section right at the top, there's a box that reads: "Show mini toolbar on selection". Un-check this box, then hit okay, and the problem should be solved. Right click should work properly again with no more text deselect. Worked for me.
What people are saying is accurate in that it has to do with tiny mouse movements, BUT this is caused by a feature that was added recently. It's that mini formatting toolbar that pops up automatically whenever you highlight something that's causing the problem. This added feature creates an overlap of systems where Word thinks you're trying to select new text and interact with the new toolbar instead of just right clicking on the text that's already been highlighted. Best way I can explain why I think the problem happens. Turning off the mini formatting toolbar avoids this confusion, and the right click option is freed up again. Not sure why this is even a feature since the mini formatting toolbar shows up just above or below the right click pop up window anyway. I guess it was added to erase the need to right click at all? This idea sounds like it would add greater efficiency, but in reality it seems to have just made things less efficient. Poor design, in other words, but not necessarily a bug. Oh well. Anyway, that's the fix everybody.