TYPO3 Core

How are they broken? You mean the missing http? This is on purpose and a valid url as it's independent from the used protocol.

not all linked images are broken, linking general works.

only the click-enlarge feature seems to be broken.

Maybe it's related to the bug produced by the fix for this bug:
https://forge.typo3.org/issues/69516
please check my note number 8:
https://forge.typo3.org/issues/69516#note-8

The double slash at the beginning of a path it's simply wrong!
(it makes the browser to try to download something like http://fileadmin/... without the domain)

I have to correct myself, click large is broken.
Frank Nägler seems to be right.

I'm having the same issue, but with a different extension: jh_magnificpopup.

I've gone through the code of jh_magnificpopup and there does not seem to be any critical stuff to change the click-enlarge markup or something, just some default booleans and classes set via TS, at least for click-enlarge. Maybe a core bug after all?