You know how if you take a Champion with a blink, say like Ezreal's (E) Arcane Shift, and you try to blink over a wall - you can actually click inside the terrain itself, but the game will say, "The Champion cannot exist within this physical space. So I will move you to a nearby 'open' location where you can exist." and it'll either "push" Ezreal on the outside of the wall and he'll get a little extra distance beyond the ability's normal range, or it'll fail and he'll end up on the same side of the wall?
Have you seen that happen? It can happen with flash, blink abilities, etc.
Basically that's what is happening. Morgana is trying to occupy the same space as the terrain so the game moves her. Is it a loophole around her Shield? Sure. Is it a bug? No. It is working at the game is intended to do.
Another way of thinking about it - if that didn't happen, if Morgana could just walk through Trundle's pillar without being obstructed in any way, but then the shield expired while inside the pillar, it could result in her being physically stuck in the space, which is why blinks and jumps work the way I described above.
/thread