It looks like we have got this problem after the update. When exactly did it happen?

Actually I had an idea...
Add this code to MediaWiki:Mobile.js and it will purge existing pages when the problem is encountered (e.g. crowd source purging the cache where needed)

if ($( '.wpb-banner-toc' ).text() === '1' ) { $.post( mw.util.getUrl( mw.config.get( 'wgTitle' ), { action: 'purge' } ) ) }

No 1 should be generated as of Wednesday. I'm still not exactly sure how this started happening but the bug was in the code from the day it was deployed and seems to impact older pages.

added to Mobile.js, but why Mobile.js? We are talking about desktop version. Should it be added to Common.js as well?

my thinking was mobile traffic was less so less traffic would hit purge urls but actually on second thought Common.js would probably make more sense but be sure to remove it if you notice any slowdown on the site and as soon as the problem goes away (I'm not yet sure of the scale of this problem and how many pages it impacts). On other bug it seems editing the template may help too!

Now I have it in Mobile.js and Vector.js. Should be enough, while adding the same line to Common.js will be redundant, right?

I have edited the template right now and also earlier this morning, but as far as I can see this did not help.

Yeh if it is in Common.js delete it from Vector and mobile as otherwise it will run twice (bad! ) :)
It will take time to see the results.

Essentially all the code does is this:
check if the toc is 1
if it is purge the cache.

@Jdlrobson: thanks for your support! At first glance, the problem has been resolved, although we still have to see how it develops after the patches in Vector.js and Mobile.js are removed