Is this perhaps deliberate, to prevent too many people trying to access updated
timezone data all at once?

More likely that server or org network access is front-ended by a proxy server
that has to scan everything before it caches it and/or allows anyone to access it.

I suppose that could be the issue. The response from https://data.iana.org/time-zones/releases/ has this header:

cache-control: max-age=86400

Perhaps what seemed like more than a day of delay was just a bit under.