Short burns are the usual culprit. Candles form a burn memory, so if the first couple sessions do not reach the edges, they keep tunneling down the center. A draft nudges the flame off to one side, and an untrimmed wick can either mushroom or drown, both of which make the melt pool uneven. Soy needs a patient first burn to set itself up for success. You can still rescue the one you have. Trim the wick to about a quarter inch, move it out of the draft, and give it a long recovery burn until the melt pool reaches the sides. If the rim is stubborn, make a loose foil collar around the top to reflect heat back in, leaving a hole above the flame. You can also gently warm the high wax with a hair dryer to level it, then let it cool flat. While it is liquid, nudge a little of the ridge into the pool to help it catch up. For next time, use the hour per inch rule on the first burn, keep the wick trimmed before every light, avoid quick stop and start burns, and keep the jar away from vents or open windows. If you want a jar that resists tunneling a bit better, try Handmade Pottery Soy Candle set which uses a thicker pottery vessel that holds heat for a more even melt and natural soy wax.
I'm with you on short first burns, drafts, and wick length being the big tunneling triggers, and your rescue steps are exactly what I'd do. That pick makes sense for your setup because the vessel keeps warmth and encourages a full melt pool, which is more forgiving in a drafty room and helps prevent wax clinging to the sides. I learned this the hard way after two quick burns near a window before switching to a warmer holding jar and giving the first burn enough time.
