This problem occurs on https://en.wikipedia.org/
I discovered that some collapsed sortable wikitables that worked correctly on 16 July 2020 (and I believe on 3 August) but on 9 October 2020 no longer showing the sortable headings.
The error occurs in both Safari 14.0 and Firefox 81.0.1 on the Macintosh System 10.14.6. I do not possess the ability to check if it occurs on other browsers or operating systems.
An example of this is the collapsed sortable wikitable in the Languages section of https://en.wikipedia.org/wiki/Allahabad_district
- It does not display the sortable headings when you click on show (though it did when the table was created on 15-16 July 2020, and I am fairly certain that it did when the table was modified on 3 August 2020)
- If you change class="wikitable sortable mw-collapsible mw-collapsed" to class="wikitable sortable mw-collapsible", then the sortable column headings are displayed.