Waaah? I actually can't remember how the Goblet shield is implemented. I know its really really weird. I'll need to look more closely into this.
From my vague recollection I swear the whole table is an object and the shield is a multi-tile invisible NPC sitting on top of it. I can't remember if the goblet as an item is actually on the table before you break the shield. If what Randoms saying is actually true across a wide range of objects then it might have destroyed the goblet section of the table while it was still an 'object' and before it had a chance to create the goblet 'item'.
I wonder if this has been a thing for a long time and no ones ever cast burst of flame directly on top of it since it was difficult to get flame clouds onto it.