Is short internodal spacing by itself ever a problem?

It means the light is too close and/or too intense. Your plant is showing signs of a magnesium chlorosis ( purple stems ). These 2 factors are the first symptoms of an excessive amount of light. Magnesium is the central molecule in the chlorophyll. The chlorophyll absorb the full light spectrum in the photosynthesis process except the green spectrum. When Magnesium gets used up during the photosynthesis faster than it can be processed by the roots, the chlorophyll doesn’t absorb the blue and red spectrum anymore and start to reflect purplish on the thinnest wall cells like stems and petiole of the leaves.