Back in June 2020, Help talk:Extension:ParserFunctions was changed from a normal talk page to a "Flow" page. There were already six subpages of that talk page that were (and still are) archived discussions split up by year (see the archives listed on the page I just linked to). When editing some of those subpages (specifically, the ones for 2009 and 2011), I found that the Table of Contents (which happened to be provided via the {TOCright} template) disappeared. Using the _TOC_ directive or the {TOC} template to "force" the TOC to be displayed didn't work: the TOC was still not shown on either page. When I reverted my edits, the TOCs were still missing. Eventually I discovered that merely purging the pages (with no edits made to them) was enough to cause the TOCs to disappear. None of the subpages have a TOC now, even though all of them except the one for 2010 had them before. So, is this a Known Issue, that the TOC will disappear from a ("normal") subpage of a "Flow" page?
Topic on Help talk:Structured Discussions/Flow
Hello
I see the TOCs on all pages.
Have you switched to the new Vector skin by any chance? If so the TOC is on the left sidebar. I'm asking as it is the most common cause of TOC disappearing these days.
No, I'm still using the legacy Vector. But the TOCs showed up for me today, too (both before and after my latest edits to those pages). I don't know what could have caused the problem, because I even tried followup null edits and page purges, and neither action resulted in visible TOCs. (Of course, they should have shown up immediately upon saving the pages, like they did after today's edits.) Weird.
Maybe the fact that I visited the pages somehow purged the cache?