500 / 500 Mbps plan results in only 500 / 100 Mbps on CODA56 modem

I activated a 500 / 500 Mbps plan this week but I've only achieved 100 Mbps upstream even after several modem reboots. Downstream is fine.

The CODA56 shows OFDMA Upstream channel 0 at 36.2 MHz with channel 1 disabled. There are no coax splitters and speed testing is done from hardwired devices or directly at the modem.

The CODA56 is capable of ~750 Mbps upstream, although I see a lower (~450) figure from Xfinity. Regardless, I should be at or near 500 Mbps.

I live in an enhanced speed area and I'm offered all the symmetric up/down plans, so I thought I could achieve rated speeds.