After looking at the forums more, I discovered a post I made in 2020 which appears to be the same issue - Last check in future? Unable to reschedule - #2 by twidhalm - I will try these steps again and see if it fixes it up.
Is this a bug I should raise on GitHub? Essentially what I think has happened is NTP or the HW clock on the host has jumped into the future, a whole heap of checks isolated to one zone got scheduled 3 months into the future, and then the clock recovered, but the checks remain stuck in the future.