I pour soy candles as a hobby and what you are seeing is classic memory ring behavior. Soy needs that first burn to reach the edges or it will remember the smaller melt pool and keep repeating it. You can coax it back. Trim the wick, shield it from the draft, and plan a long relaxed burn until the top is fully liquid edge to edge. A simple foil hat with a hole above the flame helps a lot because it keeps heat in the vessel. While it is liquid, you can gently nudge some of the high ring into the pool so it catches up. Preventing it is mostly patience. One hour per inch of diameter on the first session, then keep the wick around a quarter inch and avoid stop and start burns.
