No, that's how the card works https://scryfall.com/card/mh3/70/shadow-of-the-second-sun
{4}{U}{U} • Enchantment — Aura • Enchant player At the beginning of each of enchanted player's postcombat main phases, there is an additional beginning phase after this phase. (The end step happens after the added untap, upkeep, and draw steps.) • Modern Horizons 3 (MH3) #70 • Illustrated by Danny Schwartz • Magic: The Gathering, MTG