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.