At a guess, it may be that it doesn’t show up without normal-mapping because in that case, the standard shading hides it: With normal mapping, areas that would normally by dark may become light, and so be affected by shadows.
I think that this is a form of “Shadow Acne”; I think that, if it isn’t already implemented, this article recommended a slope-based bias to reduce this particular artefact. However, I don’t know how current that technique is–perhaps a better way has subsequently been found. Furthermore, I haven’t tried it myself, and so am not in a position to recommend for or against it. (It’s also entirely possible that I’m very mistaken in my diagnosis, of course.)