Is it normal to see purple streaks on the stems / petioles?

No, LED lights do not cause any kind of nutrient deficiency; purple streaks on stems and petioles are a normal response to exposure to ultraviolet (UV) light.

A rumor was started that LED lights cause magnesium deficiency when people noticed that some plants develop purple petioles (leaf stalks) or streaks on the stems under LED lights, but not under HPS lights. While purple coloration on stems and petioles can be one of the signs of magnesium deficiency in plants, it is also a sign that the plant is producing natural purple pigments (anthocyanin) in response to ultraviolet (UV) light. Many artificial lights (including HPS and most LEDs) don't give off UV light, so plants grown under these lights don't produce this natural pigmentation. Under these UV-lacking lights, purple coloration is often a sign of magnesium deficiency. However, when grown under UV-containing Black Dog LED lights or natural sunlight, plants will produce their full range of natural pigmentation-- it is not necessarily a sign of a nutrient deficiency.

The major symptom of magnesium deficiency is usually yellowing, blotchy-looking (chlorotic) leaves, accompanied by purple stems and petioles. When growing under Black Dog LED grow lights, unless the leaves are chlorotic, purple stems and petioles are not a sign of a magnesium deficiency-- they are a sign of a happy, healthy plant.