OK, this seems to be an odd bug!
My adventurers just got back from doing long treasure hunts, first one came back with 20 granite and 2 eggs, second came back with 10 granite and 3 eggs.
Added the granite and eggs to my warehouse - granite added as normal, but when I added the eggs, I got the message '<xx> exotic wood added to warehouse' (I forget how much wood it credited). Happened for both lots of eggs.
Checked my egg balance, and it's unchanged.
Anyone else seen this sort of behavior?