They are transparent because I'm using the alpha map as a spec map for the shader.

Thanks for pinpointing the bug, jcl.

Is there any possible way of coming up with a quick fix for this before the next engine version? A publishing company is interested in testing a beta of my game, but they need the demo by the end of the week, and the game won't work properly without this feature, and I'd rather not send them a version without specularity.