Your friend is wrong and your earlier loaves are the evidence :) Commercial yeast does NOT make bread sour - a clean, properly-timed ferment tastes sweetly wheaty, exactly like your old bakes.
Sourness has two sources. SOURDOUGH cultures - wild yeast plus bacteria - where tang is the entire point and proudly on the label. And OVER-FERMENTATION - dough that proofed too long or too warm, where the yeast exhausts its food and acidic byproducts build up. Given your loaves CHANGED without the recipe changing, this is your answer... and the usual culprit is the season. A kitchen that warmed up with the weather ferments the same recipe much faster, so yesterday's timing is today's over-proof.
The fix: watch the dough, not your old schedule. Bake at doubled-and-jiggling, use cooler water on warm days, or move the proof somewhere cooler. The tang will leave the way it arrived.
Uninvited sourness is a scheduling complaint from the yeast, not its personality 🙂
