So I disabled every single other addon I had - including WeakAuras which made the fight from the screenshot below really fun - reenabled Elvui and Elvui Options - and started a fight hoping the enemy's bars would come back during a fight.

Then, given the magical disappearance then reoccurrence, I figured it must be some other addon screwing around with things. If it worked for my warrior, it would work for me spriest. My first troubleshooting step was to copy the profile from the alt from the previous screenshot. Suddenly, without fooling around with any settings, I happily had enemy cast bars, health and debuffs. While I was waiting for a reply to this post, my issue magically resolved itself yesterday. As it turns out, my Nameplate problem hasn't been resolved.