I just noticed that I never returned to say that this problem was solved. Jim had the right answer, but I misunderstood what he was saying at the time. It is, as he said, the difference between textures being placed in the My Textures library vs. the In Model library. Now when I import a custom texture, I always switch to In Model textures first.

Hi Edgar and welcome! "Ask and get answered" as the motto of this forum says (at least there will always be somebody who triesto answer).