Late response but I got to the root of the issue. Lemmy (well, lemmy.world) wasn't being cached by Quad9's DNS servers.
I tested by using different DNS servers (OpenDNS) which worked fine, then permanently resolved it by installing unbound on my pihole and using that instead.
Can't say I've noticed owt here (outside of the ddos stuff over the weekend obviously). There was a thing where everyone had to log out and back in, including on mobile apps, when lemmy.world upgraded to 0.18.2 but that was a while ago now so probs not that if it's just started.