It can be fixed with this code:

.mw-input-with-label {
    display: inline-block;
}

But it is desirable to understand why it has display: block now.

Change 517719 had a related patch set uploaded (by Jack Phoenix; owner: Jack Phoenix):
[mediawiki/skins/Timeless@master] Fix Special:Watchlist options for no-JS

https://gerrit.wikimedia.org/r/517719

Ugh, it looks like this is only the watchlist (recentchanges is fine) and can show up with or without browser js...

In T225412#5256008, @putnik wrote:

But it is desirable to understand why it has display: block now.

The reason for the display:block in general is to put labels on their own line above text inputs and the like for consistency with how forms are done with ooui. For whatever reason it seems to be breaking the watchlist form, but not anything else I checked...

Change 517719 merged by jenkins-bot:
[mediawiki/skins/Timeless@master] Fix Special:Watchlist options for no-JS

https://gerrit.wikimedia.org/r/517719

Change 520338 had a related patch set uploaded (by Paladox; owner: Jack Phoenix):
[mediawiki/skins/Timeless@REL1_33] Fix Special:Watchlist options for no-JS

https://gerrit.wikimedia.org/r/520338

Change 520338 merged by jenkins-bot:
[mediawiki/skins/Timeless@REL1_33] Fix Special:Watchlist options for no-JS

https://gerrit.wikimedia.org/r/520338

In T225412#5418433, @Isarra wrote:

Man, without the patch-for-review I keep losing track of things that have already been reviewed/merged and resolved... >.>

And one (e.g. me) doesn’t notice the lack of the issue…

In T225412#5269395, @Isarra wrote:

Ugh, it looks like this is only the watchlist (recentchanges is fine) and can show up with or without browser js...

Also looks awful on Special:WhatLinksHere, and there is no class or id can specify it...