I add a 'background' to text via the Essential Graphics > Appearance section.
When I have a line of text that includes descenders (y,j,g,q p) the box expands downwards to accommodate these.
So when I'm creating subtitles with a background the text boxes jump all over the place and it's very distracting.
This happens even when I expand the box far past the distance of the descenders.
It's not a bug I don't think, but I'm on latest Premiere, latest Mac OS.
I'd avoid the 'background' selection and instead use a shape layer. You can import a prebuilt mogrt that will automatically adjust the width of a shape layer to the size of the text or you can find a simple after effects project online that does the same thing.
You also could use an srt file to create open captions.