See other forum posts here, and here (where the response from a ticket I created directed me to).
Long story short, “report it as a bug, the devs will fix it” (maybe allocate more server resource / an extra hamster to a zone for 5 mins), and “zones it used to affect are fixed!” (because no-one plays there anymore).
Excuse the salt, 3+ years of the same issue is unacceptable.