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any idea what could be causing this?
User error. There's no way any competent cyclist would ever be trying to shift to the granny ring until they're already on at least the third largest sprocket. You want the chain to be already pointing in the direction of the front shift before you shift, i.e. on a sprocket which is inboard of the engaged chainring for a downshift or outboard of the engaged chainring for an upshift. That way, the tendency for the chain to straighten up is helping the shift, not fighting it.
The actual cause of this being a thing which can happen at all is that Shimano spaced the rings out far enough to allow incompetent gear selections without the chain fouling the larger rings. If they put the inner ring close enough to the middle to avoid leaving a big enough gap for the chain to fall into, the chain would rub on the middle ring when you engage the inner ring and the smallest sprocket, on a 135mm hub with the shortest chainstays specified in the manual. They have to make things idiot proof, and this is why we can't have nice things.
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gbj_tester
Tijs
It doesn't shift too badly with the Sora FD-3503.
The only thing is that when I'm in the middle ring and anywhere from the fourth smallest cog to the smallest, and shift into the smallest ring, the chain will sometimes fall between the middle and the smallest ring.
This could easily be avoided by first shifting up the cassette before shifting into the smallest ring (which I should be doing anyway), but any idea what could be causing this?