Internet down when primary is down, but secondary is still up (on Multi-WAN)

I can unplug/disconnect my secondary WAN and everything continues to work like normal. However, when I disconnect the primary, the system (load-balance status) reports both connections as down, it can't ping out, etc. unless I manually disable the port (eth0).

Any idea what I've done wrong? I set up everything via 205145990-EdgeMAX-Dual-WAN-Load-Balance-Feature.