This is replying to Timo's comment below:
For the background styles we use different algorithms.
For individual lines, we use the ascent/decent of the font. That does not make sense when you are also covering all of the line gaps, so we use the actual drawing boundaries. This is as designed.

As for the issue with the transparency - you can increase the leading so they do not overlap, or you could decrease the background size or you could increase the opacity to 100%.

Thanks for the new option!

Unfortunately i have an issue with semi-transparent backgrounds. If the backgrounds of two individual lines are overlapping, you get an area with less transparency. I attached a simple picture for demonstration.

Also notice the background's height difference between the two styles. With individual lines i get a higher (but not wider) background. Strangely, if i have just one line, the background is higher if i select box background (don't know how the option is named in english). And sometimes it just sort of changes alignment but keeps the height.

Also:
Spaces at the end of caption lines should be ignored by the background.

If there is an automatic line break (and second line is shorter) there is more background to the right than to the left. You have to force a line break (hit enter) if you want to avoid that.

Hi everyone! The Motion Graphics Team just added the ability to have titles and captions to have per line backgrounds as opposed to having one box for all lines. This is now in Beta Prime builds of Premiere Pro. Anyone with a subscription can download Premiere Pro Beta Prime builds from the Creative Cloud desktop app (check the left column for "Beta Apps". A beta build will install separately to your regular Premiere Pro build so you can just test out the new feature in beta and continue your regular edits in your normal build as usual.
For details on the feature check out the post in the community forum: https://community.adobe.com/t5/premiere-pro-beta/discuss-background-improvements-for-captions-and-essential-graphics/td-p/12014927.

Hi Matt. Same here. I also am surprised that this way is supposed to be the only way to go, especially since the Open Caps looked fine so far with the adaptive caption background lenght. This here cannot be the solution so most likely Adobe will have to come up with a fix. In the meantime, like you, I cannot use Premiere 2021 for subtitles because my clients will not accept the design choice made by Adobe and not us editors or designers. Besides that problem, I think we will like the updated subtitles.

Mike, thanks for adding your information. Matt, see this thread where Trent Happel and I discuss the problem of how to move the second block.
https://community.adobe.com/t5/premiere-pro-beta/mulitple-captions-blocks-and-other-major-captions-issues/m-p/11904179#M2734

Hi Mike,
I see what you mean now - thanks for pointing that out. Unfortunately, that doesn't seem to be a practical workaround for videos with dozens of captions. The new text block default position is directly under the first text block, so you'd then have to manually click and drag (in the program monitor? I can't find another way to move it) to position things.

"Like, overnight Adobe has forced our brand look to change."

Same here. It's absolutely baffling to me that this has been forced on all users. Why on earth is it not optional?

Hi Matt -

You can have multiple caption blocks within the same caption track item. If you right-click on the caption in the Text panel, you should see "Add new text block to caption". This will add an additional block which shares exactly the same timing, but can be positioned and styled independent of the other block(s). And it renders separately, which will give you the background behavior you desire.

Mike

@Mike Berry - I don't follow. Do you mean just limit myself to one line of captions at all times? Or is there some way to have two blocks of captions visible at the same time? Because when I create multiple caption blocks at the same point on the timeline, it only allows me to have one visible at a time.

Hi Matt -

As a current solution, you can make each line its own caption block. That will give you per-line backgrounds.

Mike