I've been memory profiling arasphere.net today, and so have been taking the HS down for a few hours at a time. Afterwards, I reliably have UISIs for E2E messages transmitted in rooms whilst the server was offline. In one instance (matrix.org->arasphere.net) they recovered 5-10 mins after the server recovered. In the others (msgs from vector.modular.im and t2l.io) the UISIs never recovered. This feels very reproduceable.

The devices on arasphere.net have not changed, so this is not the same as #5095.

I'm giving it its own bug rather than losing it in element-hq/element-web/issues/2996