A fix for this was introduced in 0.18 and another one in 0.18.1. Version 0.18.1 is in release client 4 now, and lemmy.world just updated yesterday. Lemmy ml is still on 0.18.0 and doesn't have the 2nd fix yet. There are likely to be many causes to this, but consensus seems to be that federation has drastically improved since world upgraded to 0.18.1rc-4.
So, what to do? Just be patient. These are growing pains and known issues being tracked by devs and admins across instances. If Lemmy.ml and Lemmy.world wanted to defederate, they would do exactly that, instead of some silly shadow-defederation as you're suggesting.
If you want to follow the issue on GitHub, here is the link:
Lemmy.ml appears to be massively overloaded. Browsing their instance’s website results in network timeout errors. They haven’t applied the same fixes that lemmy.world has yet either. So, it’s not blocked, it’s just that lemmy.ml is currently unable to keep up with things.
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