Apple - USB devices intermittently disconnecting

Disclaimer: this is an answer under construction.

Open the  > About This Mac > System Report..., select Hardware > USB, and open every small ▼.

Select the USB bus where is connected your USB hub, and enlarge the lower part of the window to have a complete vue of all your USB equipments.

Make a screen capture: commandshift4, space, select the USB map window and click. You will get something like:

Include this screen capture within your OQ (original question).

Verify that the sum of the Current Required (mA) of all the equipments connected to the same internal USB hub don't exceed the Current Available (mA) for the hub. This value might be different depending on the USB port available outside and what is already connected on it internally.

As a basic simple rule, avoid the external USB port which is on the bus where is connected the FaceTime HD camera (Current Required (mA) = 500 mA). For example, on a MacBook Pro, I discovered through trials that my FaceTime HD camera is on the bus with the USB external port nearest from the front.

To check this hidden cabling, just use your external mouse.

Connect it on port nearest from the front. Hit commandR in System Report... to refresh the USB connection information. Note on which bus is this external USB port.

Repeat the same operation for port farthest from the front.

After trying every possible combination of hubs and configurations I could try, and with the very helpful guidance of daniel Azuelos (other answer) it turns out that in my scenario, everything started working seemlessly when I attached a USB 2 hub to the USB 2 port in my Macbook Pro (on the left), and attached only USB 2 devices to that hub.

I attached the USB 3 hub to the USB 3 port (on the right) and attached only USB 3 devices to that hub.

It seems when doing this everything stays connected and stable.

In particular the problem seemed to be arising when having USB 2 devices attached to any hub on the USB 3 port (regardless of whether the hub was USB 2 or USB 3). I don't know why this behaviour occurs, but keeping 'like with like' has worked in my situation.

Interestingly, the USB 2 hub is unpowered.

I do audio production meaning I sometimes use a LOT of USB devices, including eLicensers, an audio interface, synthesisers etc. And the USB 2 devices just did not like being on the USB 3 port.