It's not an unknown problem. Some people interpret the odor as soapy. Some as fishy. One proposed cause is failure to thoroughly mix in the soda, either lack of stirring or lumpy soda. But the real question is why the odor. I have a notion about that, and I think there really are two different odors that can form.
Alkali and fat, under the right conditions, make soap. It's called saponification. And alkaline water environment can turn a body to soap. The most common soap making technique is the "hot process." In hot process, the fat moves through the intermediate step of fatty acid to soap very quickly in the pot. There's a cold process, too, that takes longer. Soap making can result in failures to complete the conversion.
Soda bread dough is hardly an ideal mixture for saponification, but the components are there, with enough heat to begin it and enough other ingredients to make it fail as a soap recipe. Every soda bread recipe has soda and fat of some kind. You certainly can make soap with butterfat. An excess of soda or areas of concentrated soda acting on the fat, especially if the dough is altogether poorly mixed and soda lumps exist intimately with pure fat lumps. Fatty acid decomposition in some vegetable oils is a significant storage problem, and one of the bad effects is fishy odor. Soda is not a strong enough alkali to efficiently complete the process. So, it's not unreasonable that a forming of fatty acid and subsequent decomposition, accelerated under baking conditions, could produce fishy or soapy odor. Which you produce may depend on just how far the saponification process proceeds, when conditions are present for it to begin.
The solution is to measure the soda accurately, and mix it well, sifting if needed.
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen