Thank you all for the feedback!
So we learned here that these DNS issues are happening in the DNS resolvers of various ISPs, notably ones in the UK. Switching to alternative DNS resolvers (Google works, but Cloudflare or Quad9 probably do too) bypasses this issue.
I’m not in the UK so I can’t really check. But iFastNet is, so I have brought this up with them too. Hopefully someone in the company is on an affected ISP which should give them a good starting point to investigate.