you are from lemmy.antemeridiem.xyz, which means beehaw.org still federates with your home instance. this only affects users of sh.itjust-works/lemmy.world <-> beehaw.org. also, i suspect already synced threads wont disappear, but new content won’t be synced anymore (this part is just a theory, if someone who knows this could weigh in i would appreciate)
What do you mean synced posts? If instance 1 and instance 2 ar federated, do they each have a copy of the other’s data?