I looked up this issue and found a post on the forum here from 2017 by @AndersOfDenmark.
An Adobe employee said they were aware and working on a fix in 2018.
Using the latest Premiere Pro v23.00 (Build 63) and the problem continues.
The issue is the background for caption/subtitle shifts from line to line depending on the length of text.
Here's an example. These are back to back subtitles and I added a guide so you can see the shift.
This is super distracting when watching in real time.
Adobe, any updates on this bug?