Titles "jump" iMovie 10.2.1

I'm putting titles over a background in a short video and most of them are "jumping." That is, in each 4-second title, the words appear and then a second or two later, the words jump into a different position. They're still in order, but they may move from one line to the next. I can't use the video with this happening, and it's happening for nearly every one of the six titles in my 1-minute promo video. Any thoughts on how to fix this?

Big Sur 11.0.1