@bwang The code is already putting this to display: block:

@media ( max-width: 1399px ) {
	// By default, limited width client pref should be hidden unless the breakpoint below is reached.
	.vector-feature-limited-width-clientpref-0 #skin-client-prefs-vector-feature-limited-width {
		.skin-client-pref-exclusion-notice {
			display: block;
		}
	}
}

Source: https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/skins/Vector/+blame/dd9a26f9ea3d180cf07c74d29960c6d14d878a7b/resources/skins.vector.styles/components/Appearance.less#26

I don't see how this would have end(ed) up in the incorrect styling in the screenshot, both predecent sibling ul.vector-menu-content-list and span are display: block.

Tested at 1126 canvas width:

Do you have further steps to reproduce?

@Volker_E sorry this wasnt clear in the original task, but the appearance menu has to be unpinned, i.e. in the vector header. try clicking "hide"

Change #1318209 had a related patch set uploaded (by Anne Tomasevich; author: Anne Tomasevich):

[mediawiki/skins/Vector@master] Fix appearance menu exlusion notice layout

https://gerrit.wikimedia.org/r/1318209

Change #1318209 merged by jenkins-bot:

[mediawiki/skins/Vector@master] styles: Fix appearance menu exclusion notice layout

https://gerrit.wikimedia.org/r/1318209

Change #1318735 had a related patch set uploaded (by Bernard Wang; author: Bernard Wang):

[mediawiki/skins/Vector@master] Generalize flex-wrap style for .vector-menu

https://gerrit.wikimedia.org/r/1318735

Checked on enwiki beta - the fix is in place. The gif below shows the correctly formatted label for different screen width.