I can confirm. At first I could not reproduce, then I put a cauldron in a hole in the ground and all the inner sides disappeared. This seems to just be a mistake whereas the inner sides are classified the same as the outer sides, so they are hidden alongside the parts which should be hidden. I can sort of fix this by adding "cull": false to all faces not at the outermost layer, but then they always render even when the block is completely hidden.
I can confirm. At first I could not reproduce, then I put a cauldron in a hole in the ground and all the inner sides disappeared. This seems to just be a mistake whereas the inner sides are classified the same as the outer sides, so they are hidden alongside the parts which should be hidden. I can sort of fix this by adding "cull": false to all faces not at the outermost layer, but then they always render even when the block is completely hidden.