Steps to replicate the issue (include links if applicable):

• Go to https://en.wiktionary.org/wiki/Reconstruction:Proto-Sino-Tibetan/s-n(u/a)m
• Notice that there is a slash in the pagename.
• Look at the headword, just below the Noun header.
• It says *a)m when it should say *s-n(u/a)m.

What happens?:

Subpage handling in the Reconstruction space (and possibly others) is broken, chopping off everything before the last slash.

What should have happened instead?:

The MediaWiki subpage function mw.title.new("Reconstruction:Proto-Sino-Tibetan/s-n(u/a)m").subpageText should return "s-n(u/a)m" instead of "a)m". This is a recent regression and used to work. It does not seem to happen in the mainspace and may be specific to the Reconstruction namespace.

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):