If you place a shield generator with a shape card, shield will be only generated on non-solid blocks like air, water, etc. And it won't fill the air blocks when the terrain is changed, you need to turn it off/on to fill those new air blocks.

That means that if you plan to protect your house with a shield, anyone can excavate under the shield to get inside.

It should extend the shield to the new air blocks like the MFFS mod does.

I'm afraid that that would be pretty hard to do efficiently and would also break builds that actually depend on the shield not forming where there are blocks. That's a good way to make holes (doorways) in a shield for example