Charging issue on 2020 M1 MacBook Air

For the last 2 days (coincidentally since updating to Sonoma), my MBA intermittently refuses to charge when power is applied. By that I mean that the battery icon at the top of the screen does not show the powerplug overlay, and the beep doesn't sound when I plug the power in to either of the 2 TB3 ports. The screen is currently showing 78% charge, and the System Setting option for Optimised Battery Charging is turned off.

If I plug the cable and power supply from a 2019 Intel MBA in, it starts charging immediately on either port.

If I then pull the 2019 power out and plug my 2020 power in to the connector nearest the front, it again charges, but the rear port still refuses to operate.

Looking at System Information > Power > Battery Information, it shows State of Charge % as 71, different from the 78% shown on screen. Cycle count is 273, Condition Normal, and max capacity 93%.

However, whenever I plug the charger in and it shows on the menu bar that it is charging, the AC Charger Information in System Information shows No to Connected and No to Charging.

Now I have just plugged my 2020 power in to the rear TB3 port and the Menu bar on screen shows it is charging OK, and it beeped, but the System Information still shows No to charging and No to Connected. I leave it on charge for a few minutes and the charge level does go up.

So my 2020 power brick is working, and my 2020 cable from power brick to TB3 port is working, and has no sharp bends or kinks in it.

But there is an intermittent failure somewhere.

I have 2 docks to provide other ports, which I use a lot for development. I purchased the second because I thought the first had failed, but both seem to be affected by this intermittent feature.