hi so something needs to be put in place on skyfactory 2.5 to help claim and protect your builds in the nether and other farm worlds for Skyfactory 2.5, i had a building made of wither proof blocks to kill withers in (cause you cant build them on the islands sadly) and in its place i came back today to do some stuff with it and found this in its place Screenshot by Lightshot i think either Wither killing / summoning needs to be allowed on your island or a claim / protection system needs to be put in to help protect your builds where wither farming is allowed
My only advice is to tear down your build in the nether after you're done using it. Should be easy since vein miner is a thing
At the moment there is no need to put claims on the nether, there are some tools that help you protect your area in nether without the need of a claim, skyfactory 2.5 have rftools shield system and thaumcraft warding focus will help you with this.