If whatever math determines the weather is based on server up-time, or is a running simulation, etc, it’s quite possible for the issue to live in some variable that lives in memory on the server, and for it to be resolved by a server restart without determining the actual cause of the issue.
Considering that the reproduction steps are “let the server run for 3 months without a restart”, it could be rather difficult to debug on a dev machine. They should be able to work on it on the prod server, but this issue came up after they stopped actually working on the game so 
It looks like the last restart was probably Dec 17, 2021.
3ish months sounds about right for how long it is between this happening relative to known server restarts.
Hopefully @james can address this relatively quickly. I know last time it took quite a while to resolve. I don’t know if it’s possible with their server setup, but it is possible to automate server restarts in most cases.