If this is a return leg and the gold was the plat's companion on the outbound, the gold may be treated as a plat companion again automatically. But that doesn't explain why a plat companion would be higher than a plat, or how someone split them.
I had this happen on a IAH-SAN-IAH trip. Gold sponsors no-status IAH-SAN, on SAN-IAH and then no-status automatically appeared on UG waitlist at t-24h auto-checkin at the expected location for a Gold companion.