I find it convenient and pretty cool that you can post to Lemmy from Mastodon by tagging the Lemmy community at the bottom of the Mastodon post.
I noticed a little over a month ago, this stopped working for me when I’d post from my fosstodon.org account and tag any community hosted on lemmy.world, and upon inspection, I see that Lemmy.world defederated from fosstodon.org
Threads.net isn’t even defederated by lemmy.world, it’s under the linked instances list, but fosstodon.org is blocked? What was the reasoning for that?
Fosstodon are just as vigilant against that kind of stuff, so I’d be surprised if it went under their radar and was posted to lemmy.world, but none of the other big mastodon instances had that problem.