Lemmy.world has been acting strangely since yesterday's upgrade, and has also been getting hugged by Reddit refugees.
I think it's having issues right now seeing posts elsewhere in the fediverse, and sharing posts made on lemmy.world with other servers.
Using an alternate account on lemm.ee I can see a lot of lemmy.ml posts in here that aren't visible on lemmy.world, and none of those instances seem to be defederated from each other.
I expect it's just growing pains and will be resolved soon.
tl;dr You might want to make an alt.