Samsung Galaxy S6 (SM-G920W8), Android 5.1.1, MCPE 0.14.0 build 4
Tested in single player creative, expect the same results elsewhere.

In build 4, item frames can attach to sides of blocks that are not a full block. For example, the low step or sides of stairs, sides of slabs, even the sides of redstone dust (see MCPE-13165).

When you do that, you can see at least part of the back of the item frame.

When you then put an item in that frame where the item sticks out of the frame (e.g. fence, block, TNT, etc. In contrast, not a bucket, sword, glowstone dust) you can see the other half of the block sticking out the back of the item frame. This means the item frame (a block entity) takes up space outside of it's block. That seems bad.

But if that isn't a bug, then maybe it's a bug that the item frame can be placed on the sides of blocks that are not a full block height.

In the screenshots, I've shown how TNT goes through the item frame and this is visible when the item frame is attached to the low step on stairs or to redstone dust. Ignore the activated redstone dust (there's a torch underneath you can't see, at the time I was testing if item-framed-TNT could be activated by redstone. It can't. Sorry.)