There is no bug here. It's like that because it's separating itself off from the bar that contains the checkbox.

I respectfully disagree, please check the regular plugin update notification in case it's the last row (border-bottom / box-shadow is removed from the last td):

I'm not sure what your last comment is designed to show. Even when there's no update listed for the last plugin, there's a clear difference in the border between that plugin and the checkbox.

Do we understand eachother correctly?
I mean the double line, it should be a single line:

Yes, we understand each other. And, no it shouldn't be. As I say, it's there to differentiate itself from the area below, which is also done even when there is no update (albeit the differentiation then uses a slightly different method).

Have removed my previous comments, because of info underneath:

I understand what you mean now; 2 lines because separate boxes.
My PR in fact removes the third line. Similar to how WP does it.

I think the CP "block-related functions" tr isn't always targeted by the current CSS, because a script tag is added after each CP "block-related functions" tr. So all tr's after that script tag aren't targeted by the current CSS.

Changing last-child into last-of-type fixes this.

The current CSS is targeting the element correctly so far as I can see:

Perhaps there's something else happening on your site, @Guido07111975 ?

It only happens with the "block-related functions" tr, did you test with that?

I'm not sure what you mean. I have Blocks Compatibility switched on.

Only happens if the "block-related functions" is the last one in the list.
Tried to explain why, in a previous reply. Caused by a script tag that is added. That script tag kind of breaks the tr list, resulting in the CSS not being able to target any tr after that script tag. So if there's more than one "block-related functions" tr, the last one is not being targeted with the CSS.

I understand everything except what you mean by "block-related functions". As you can see, there's no script tag in the way on my site with Blocks Compatibility on.

You may have turned it on, but do you also have the "block-related functions" notification at your plugins page?

Maybe I should close this, as it only happens when having more than one "block-related functions" ánd one of them is the last one in the tr list. So will not happen often.

You may have turned it on, but do you also have the "block-related functions" notification at your plugins page?

No. Why would I?

Only happens if a plugin has block related functions. Guess you don't have such a plugin installed. And I understand why.

My last try to make myself clear.
With a minor tweak you can make the "block-related functions" tr display at plugin page without having to install a plugin containing block-relating functions:

1. Settings > Reading: set Blocks Compatibility at Troublehooting.
2. Remove line 55-57 from file class-wp-compat.php.
3. Go to the Plugins page: each plugin now has the "block-related functions" tr (check screenshot). And the last plugin in the list has a "third" border/box shadow.
4. Inspect the source code and you will see the script tag after each plugin (check screenshot).

But if a plugin for example has an update available, you will see there's no "third" border/box shadow added.
Again, this is only relevant if the plugin is the last one at the plugins page.

If we replace the script to inside the tr (so 1 line up), it still adds the update class to the main tr and also solves this PR (without the CSS fix from this PR).