I have had this randomly happen on my AC88U. As others have said, the 5GHz completely stops broadcasting -- all 5G clients lose connection simultaneously, and no client or scanner can see the 5GHz SSID -- but wired and 2.4GHz clients continue operating normally.
It has happened with both stock Asus (380.7266 and 380.7378) and Merlin firmware (380.65_2, 380.65_4, and 380.66_beta4). After flashing each firmware, the router was fully reset and setup from scratch. Router sits on a laptop cooler, so radio temps are <50c, and cpu temp is <70c.
On Merlin, I have tried toggling "airtime fairness" and the various beamforming options, but these have had no effect. There are no other 5GHz networks in my area, and no wireless phones/etc. using 5GHz band. The router (and modem) are on a sine-wave UPS.
There are typically about (9) 5GHz devices connected, including iPhones, iPads, Dell and Lenovo laptops, and some Amazon devices. All of them simultaneously lose connection when the AC88U drops the 5GHz signal. All of the wired and 2.4GHz devices (quantity of 22) all continue working normally.
Sometimes 5GHz will be stable for several days, while other times it fails within a few minutes or hours.
The last failure occured after it had been stable for 6+ days. It went like this:
1. All 5GHz clients lost connection.
2. The router GUI had showed nothing unusual in the syslog. The wireless log still shows clients connected, but all data (like speeds and signal strength) are static and do not change at all between refreshes of the page.
3. Toggled the 5GHz radio OFF, then ON. Operation returned to normal.
4. That lasted about 10 minutes, then it died again (same symptoms as 1-2).
5. Rebooted the router. Operation returned to normal.
6. That lasted for 8 minutes, then it died again (same symptoms as 1-2).
7. Powered off router completely, then re-powered. Operation returned to normal.
8. That lasted for 6 minutes, then it died again (same symptoms as 1-2).
9. Powered off router again, then re-powered. Operation returned to normal -- and has now been stable for 48+ hours.
Note that there were NO changes to any settings there. And even after a power reset, failure may come in minutes (as in #7-8) or operate fine for days (as it has after #9, or as it did before #1).
At least for me, it seems pretty "random", and not associated with any particular firmware. Whenever I make a change and it is stable for several days, I become hopeful... Only to have the problem occur again, sometimes after nearly a week of stable operation. I'm most inclined to point at the wireless driver, since I have seen it reported with multiple hardware models.