Upon further investigation, the explanation seems to be a lot simpler.
Since Hari Raya, when there was an attempt to send a mass greeting email, the mail queue of MUW has been jammed.
3000+ emails being sent out at one go seems to be the cause.
Since then, emails have been constantly sent out late (some by several weeks) or not sent out at all.
The mail queue has been cleared and the queue rules have been relaxed to ensure jams don't happen again.