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

UPDATE (Sept 30): Not a client issue. The RBE971's 5 GHz radio fails on channels 36/40 for every client; satellites and a different router in the same spot are fine. NETGEAR has declined an RMA.

A lot of testing since the original post. The short version: my original theory (WiFi 7 / mixed-security interop with the Intel AX211) was wrong. It turned out the "good" IoT-SSID result in my first post was actually through a satellite, not the router. Once I controlled for which Orbi unit the laptop was connected to, everything lined up differently.

What I found

• Every 5 GHz client on the RBE971 router on channel 36 or 40 is bad: ping to 192.168.1.1 avg 150–430 ms, spikes to 1.7–3.7 s, occasional loss, throughput 5–40 Mbps. This includes my Intel WiFi 6E laptop, a second laptop with a Qualcomm WiFi 5 adapter (avg 301 ms, max 2140 ms, 100% signal), and a Sony TV (buffering that never happened before).
• The same clients on the same SSID through either RBE970 satellite, same channel: avg 5–6 ms, max 14 ms, 0% loss, 990 Mbps.
• The router's 2.4 GHz radio (avg 28 ms) and 6 GHz radio (iPhone/Mac 1,100+ Mbps) are fine. Wired to the router: 1–4 ms.
• SSID and security don't matter: the IoT SSID (WPA2-only) through the router is just as bad.
• Client settings don't matter: 20 MHz width, 802.11ac mode, power saving off, disconnecting the MLO iPhone and Watch, all no change.
• Factory reset of the router; tested on defaults before restoring anything: avg 329 ms, 1 lost. No change.
• Rollback to V12.1.10.12: avg 232–426 ms, max 3.7 s. No change. The router's transmit rate to the client had dropped to 52 Mbps while the client's was 490. Back on 12.1.11.15 now.
• Changing the 5 GHz channel to 44 (suggested by Netgear phone support) immediately returned it to normal. Avg 3–4 ms, max 12 ms, 1,084 Mbps down. Auto picks 40, so Auto reproduces the fault.
• Back to channel 40 with a nearby printer and Arlo base station unplugged: avg 1117 ms, max 3777 ms, 16% loss, worse than the previous week. Back to 44: avg 3 ms.
• Control test: a Linksys WiFi 6E router placed in the RBE971's exact location, Orbi powered off, same laptop on channel 36: avg 5 ms, 0% loss. A scan from that spot shows no other 5 GHz networks on 36–48.

So: one radio in one unit has lost the ability to operate on the lower 5 GHz channels; it survived a factory reset and a firmware rollback; it's getting worse; and identical radios in the satellites, plus a different brand's radio in the same location, are fine on those channels. Channel 44 works today.

NETGEAR's response

Support's position (case 49892824) is that because the router performs normally on channel 44, the radio is "functioning"; the problem is "channel conditions" or "environmental factors that may not always be visible during standard WiFi scans." I should set the 5 GHz channel to Auto. I replied that the Linksys control test rules out the environment, the satellites rule out the channels, and Auto selects 40, the failing channel. Waiting on escalation.

Questions for the community

1. Has anyone seen an RBE971 (or other Orbi) develop a channel-specific 5 GHz fault like this? A corrupted calibration table or internal self-interference from the backhaul/6 GHz radios seem like plausible mechanisms; I'd be interested to know whether either has come up before.
2. Any advice on getting a hardware case escalated past the "it works on another channel, so it's fine" script? The hardware warranty has ~145 days left.

Happy to share the full logs (ping outputs, netsh interface dumps, WLAN report) if useful.