So, not sure if this in intended, but, like I've said before, better safe than sorry. So, apparently, Shulker Boxes can't have Item Frames placed on top of the lid when placed on it's side. This is because after you place it, when you open the Shulker Box the Item Frame will pop off. However, the Item Frames can still go on the sides of the Shulker Box and it will open just fine without them popping off. So, not sure if this is intended, but it is kinda annoying, so here' how to recreate:
1. Acquire Shulker Box.
2. Place Shulker Box on it's side, so that the top of the lid faces either the X/-X or Z/-Z axis.
3. Place an Item Frame on the three exposed sides of the Shulker Box.
4. Open the Shulker Box.
5. Observe that the Item Frames on the sides of the Shulker Box stayed on the Box, but the one on the top of the lid popped off.