2866Vac dropping 5G SSID every day/uncommanded channel change

CORRECTION: After further investigation, my initial problem description was incorrect.

Symptom is that my iPhone and iPads become disconnected from my 5GHz SSID and are unable to reconnect. If I manually force a reconnection attempt, the iPad reports 'Incorrect password'.
The 5GHz setup specifies Channel=60.
Two different WiFi tools show it is actually broadcasting on Channel 112/Channel width 20 MHz. Yesterday evening, it was broadcasting on Ch100.

A reboot or WiFi disable/enable reverts to Ch60 and devices can connect... until another uncommanded channel change occurs.

The actual channel numbers are not the issue; if I manually select e.g. Ch100, my devices connect. 
But when the router changes channel itself (strange behaviour because the Auto option is no longer an option after then last firmware update), devices are disconnected.

Running 4.5.2.2.BT, which was installed mid-February. Why is this fault only happening recently (last 7 days).
What to do?

Mark

P.S. Also discovered that the press-button to toggle on/off the 2.4G/5G wifi...does nothing. Press and long hold sems to activate WPS... but the button does nothing else

Channel 60 is one of the DFS channels - therefore it's subject to change if the router detects anything else on that channel.

If you stick to 36/40/44/48 (with channel width 20Mhz) you'll find it doesn't happen.

https://www.draytek.co.uk/support/guides/the-5ghz-wireless-band

DFS could indeed explain the uncommanded channel change, but surely that's no reason for connected devices to be disconnected and fail to rejoin?

I've been using A-Upper for many years without any problems.

Also, would I not see something (WLAN) in the syslog?

You'd like to think so, but finding these things never seems to be easy.

I'm pretty sure I've read something about Apple devices being finicky with Draytek 5G - especially that 'ac' chipset which is as flawed as they get unfortunately (see some of my other posts on that).

Indeed, a few years ago when I used AP903's (or even earlier iterations?) Apple devices would frequently refuse to connect on 5GHz with the "incorrect password" error, even though it was correct. No such issues with ax devices.

It hadn't occurred to me that my AP903's would suffer the same issues... but of course they would.
Thanks for the alert.