A server running an old add-on now says so

The portal runs on several servers at once, and add-ons update themselves in the background. An update is written once, for everyone, and each server picks it up the next time it restarts. That is deliberate: swapping code underneath a running server is how you get half-updated behaviour.

The gap was what happened in between. A server that had not restarted yet kept the previous version of the add-on — correctly — and reported itself completely up to date while it did. The check behind that report asked only "is this add-on running here at all?", and the answer was yes; it had simply never been taught to ask "is it the version everyone else agreed on?".

So two servers could serve the same portal from two different sets of add-ons, for as long as neither restarted, with nothing anywhere saying so. On one visit a page would look one way, on the next visit another, and every health signal read green. It also confused the portal's own build machinery: a page type rebuilt on one server, then rebuilt again on the other, each undoing the other's work — and where an add-on was missing from one server entirely, a page type that had been working simply stopped.

Each server now compares what it is actually running against what has been published, version by version, and reports the add-ons it is behind on by name. The remedy is unchanged and was always the same one — a restart — but it is now a stated to-do instead of something you had to guess at.

Two things it deliberately does not do. It never asks for a restart it cannot promise will help: if an add-on's newly published files are missing, it says re-install instead, because no number of restarts would fix that. And where a server genuinely cannot tell which version it loaded, it stays quiet rather than raising an alarm nothing can clear.