What were you trying to do?:

Add cateogries to https://en.wikisource.org/w/index.php?title=Category:WikiProject_NLS and then view them in that category.

Previously I was adding these as the relevant [[Category: ]] directly (under the page-list field) but more recently I moved to adding it via the category field of the edit form provided.

What was the observed mis-behaviour:

If I add the relevant category manually ( And thus inside the MediaWiki:Proofreadpage index template ) it sorts correctly in the on the Category: page.

If I use the Category field provided by the edit-form , it categorizes the item under W, even if the nominal page name doesn't start with a W. ( I think it's using W because that's the first letter of the category name I was trying to use.)

What was the expected behaviour:

Normal (default|sorting) based upon the pagename.

I asked about this on English Wikisource https://en.wikisource.org/wiki/Wikisource:Scriptorium#Mis-sorting_behaviour and was advised to raise a ticket here..