There are lots of federation issues at the moment with 0.17 and 0.18. Both world and ml are running the latest possible versions as well -rc. There might be issues again with 0.18.0 and 0.18.1.
On top of that we have had some instabilities that I’m looking to fix this week.

These could have attributed to the issues but my money is on both of them using the latest versions. Talk on the matrix channel was around problems with them federating so I’m not surprised we’ve seen an issue with it.