I tested some situations when a shulker box is blocked by trapdoor and found in current version:
A shulker box CAN be opened when be blocked by a top trapdoor and CANT be opened when blocked by a bottom trapdoor or any other opened trapdoor.

So:
A downwards or horizontal shulker box CAN be opened when blocked by a top trapdoor even if it seems that the shulker box is blocked directly
And it CANT be opened even if the trapdoor is in the opposite of the shulker box and there is enough space to open it.

I think it should be:
A shulker box CAN be opened as long as the trapdoor is placed in the opposite of it,else it CANNOT be opened