I started to experience this a few weeks ago: the “All Incidents” page (i.e. https://mycompany.pagerduty.com/incidents) got “stuck” in showing the latest incident to be in February. If I logged in with a brand new/clean browser profile, the latest incidents would show up as expected. First I suspected the reason to be some browser extension, but it turned out to be data stored in Local Storage. Once I cleared everything from Local Storage (for the given site/domain, i.e. mycompany.pagerduty.com), the latest incidents showed up.
I’m using Google Chrome (currently v91.0.4472.77) on Ubuntu, but there were colleagues of mine using Windows with the same symptom and the same solution worked for them.