iMovie titles option - upper case only?

I'm running a much older version of iMovie 10 than you, so cannot test whether the iMovie 10.3.1 font displays are different. However, some fonts with some titles have all caps. It varies with the font and title. For example, Avenir Medium and Helvetica Neue display all caps with some titles but have the option for upper and lower case with others. For the most part nearly all of the fonts and titles display upper and lower case when the Caps Lock keyboard key is not engaged. So you might try changing fonts and titles, and making sure that you Caps Lock key is not engaged. See if that makes a difference.

If the above does not explain it, then try resetting/deleting the iMovie preferences. To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

If still no luck, a redownload of iMovie might solve it.

-- Rich