I was using iMovie today for the first time in a very long while (years) and noticed this problem as well.
The quickest workaround solution I found on my mac mini was as follows (sounds long, but only takes a minute):
Drag the Title box that has the effect you desire into your timeline as usual.
Open the TextEdit app separately. Type the word (or words) you want in lowercase (or upper/lower combination) in this app.
Then "cut" the text from the TextEdit app.
Return to iMovie app. Click on the all Cap text default in your editing panel until it highlights all the words. Then "paste" right over that. (The text you typed into the TextEdit app and then cut should paste directly here and eliminate the ALL CAPS default text simultaneously).
You should also be able to revise the new text as desired (upper or lower, change font, etc.).
I imagine there are other apps or areas you could type, cut and paste from, but the TextEdit app on my mac mini was the first one I tried and it worked ok.
Hope that helps.
Good Luck.