RBE971 V12.1.11.15: Intel AX211 client severe latency on main 5 GHz SSID, fine on IoT SSID

Orbi 970 (RBE971) V12.1.11.15_5.4.12: Intel Wi-Fi 6E AX211 client gets severe latency on main 5 GHz network only

Since roughly Sept 22–23, my Dell laptop (Intel Wi-Fi 6E AX211, Windows 11) gets 5–40 Mbps and erratic latency on the main SSID. Pings to the router (192.168.1.1) average 160–270 ms, with spikes to 1.7 s and occasional timeouts, at 91–93% signal while sitting next to the router. The device worked normally on this same setup until then.

Isolation tests, all from the same spot:

• Main SSID, 5 GHz (WPA2+WPA3 mixed, Wi-Fi 7 features), channel 40 or 36: bad, as above
• IoT SSID, 5 GHz, WPA2-only, same channel 40: ping avg 5 ms, max 17 ms, 0% loss
• IoT SSID, 2.4 GHz: normal for 2.4 GHz (~125 Mbps, ~20 ms)
• Phone hotspot: ping avg 3 ms
• Wired to the router: normal
• iPhone (5 + 6 GHz) and Mac (6 GHz) on the main SSID: normal (1,100+ Mbps)

Rebooted the whole system, changed the 5 GHz channel, and reset Windows adapter settings; no change. No Intel driver update on the laptop. This looks like an interoperability problem between the current firmware's main-SSID configuration (Wi-Fi 7 / mixed WPA2+WPA3) and Intel AX2xx clients. Was anything in this area changed in V12.1.11.15?