I assume you checked your chests, etc at your home base to see if you had just stashed it somewhere?
Have you checked the merchants you usually trade with? If you accidentally sold it they should still have it.
If you are using a 2 handed weapon, make sure the dragon shield is not in your equipment window. Your character will still auto-equip a shield when picked up if you have a 2 handed weapon (which can be annoying), though the shield slot will be darkened (making the shield hard to see).
There is a way to replace quest items by file swapping, but unfortunately any equipment gotten this way would have its stats corrupted. Such a replaced dragon shield might not be much good as a shield (it could end up classified as a staff with a damage range of 2-6, or something) but it would still work to complete the quest to assemble the dragon armour.
Lost Items - guide and directory of inventory filesActually, since the dragon shield is on the ground when you find it, the above procedure may work (I have not tried this) to get a new shield if instead of backing up and replacing the inventory files you get back into the castle and do so with the world.x0 file (using the default '..
\Divine Divinity\Run\main\startup.v2\world.x0 file to get the shield).
If you have already completed the dragon armour quest, you can use
DAD's item editor to modify a different shield to match the dragon shield's stats, as best as you recall.
There is a bug which can potentially destroy items, if you become over encumbered.
weight bug and disappearing chests