I made these over the weekend and they plumped up while baking (like the ones in the photo above seem to have done). Each day since then, they've...

I agree with Dr. B. that the quantity of baking soda is on the high side. Baking soda is proportional to the amount of acid present. It adds to leavening reacting with acid(s) present, creating CO2, a gas which expands in the heat of the oven causing baked goods to rise. The classic ratio is 1/2 teaspoon of baking soda for every 8 ounces of acidic ingredient(s). In this recipe, between the molasses and sugar source, you have 8-10 ounces of acids (depending whether you use a wet or dry sweetening source among the three options you're given), requiring 1/2-3/4 teaspoon of BS. The extra teaspoon won't add extra leavening, but will certainly produce a "soapy" taste, which may be the flavor you don't like about the cookies. I think your hunch that the recipe is flawed is correct.

I noticed that there's a lot of baking soda in the recipe (1.5 teaspoons) relative to the amount of flour. Even for a cake, when there are acidic ingredients like molasses, you only need 1/4 teaspoon per cup of flour. I think that's why they've plumped up so much.

Well, this is good to know. Thanks for the quick reply! I made Amanda's mother's cut out cookies (which use only white sugar) the same day, and they are so crisp that they're rock hard, though they've been stored in exactly the same conditions. I'm actually not that crazy about the flavor of these gingerbread cookies -- personal preference, not that there's anything particularly wrong with the recipe -- so I'll probably go back to my Americanized Lebkuchen, when I don't require a vegan cookie. They're not crispy, but are more sort of chewy-hard. ;o)

I took a look at the recipe, and all three of the sweetening options given are very hygroscopic, meaning that they will literally pull moisture out of the air. That's one of the reasons high fructose corn syrup is such a darling to commercial bakeries: it give those super-sticky muffins a loooooooong shelf life. Given the relatively high humidity where you live, softening of these cookies is inevitable unless you hermetically seal them. Beautiful photo, by the way.