I edited this answer and posted this answer but they didn't bump.
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$\begingroup$ Is it possible that you are viewing some other tab, and not active? When I look at questions tagged (extension-field) and choose the tab displaying recently active questions, I see it at the top. $\endgroup$ – Martin Sleziak Apr 5 '17 at 11:49
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$\begingroup$ Thanks! I also see it at the top when I click on your link, but I don't see it if I go to math.stackexchange.com and choose the "interesting" option among the options "interesting", "featured", "hot", "week", "month". I just wonder why. PS: here is the link: math.stackexchange.com/?tab=interesting $\endgroup$ – Pierre-Yves Gaillard Apr 5 '17 at 11:57
After the clarification in the comments it seems that it is not a bug - whether you see recently active questions depends on the way you view the site.
- If you go to https://math.stackexchange.com/questions then you have several tabs to choose from. If you want to see the question which have some recent activity, you have to choose the active tab.
- IIRC active tab used to be also here: https://math.stackexchange.com/. However this was changed not so long ago. See here for more details: Question update notice not showing any more and Proposal: make the "interesting" tab the default on Math.SE homepage.