Playing on 2.1.2, Flat Platform P, and I cannot break Oak Shelves. Tried an axe, pickaxe and empty hand for ~1 minute each, shelf was still there. Went in creative and punched it away
No changes to modpack, KeepInventory on, doFireTick false.
Another potential issue (though I can't think of any fixes) is that it's possible to duplicate... everything, by switching between online and offline mode. The inventory is preserved, but the quest progress is kept separate between the online and the offline players. If you use different names each time you play offline, you can get basically unlimited quest rewards, especially from the simple craft or consume quests that only take a bit of infrastructure.
Edit: Island is version P, not O.