After I had the issue in the topic and wasn't able to find any solution back 4 years ago or so, I found the only solution was to buy an UFX+ and connect with Thunderbolt instead of USB.
After a few years and moving to a new studio space I wanted to check if anything had improved so I've updated all the drivers (both USB and TB) and firmware and found that the issue is still there.
I'm sure I'm the only one getting this as it's impossible to accept this is affecting more than one, however I thought about sharing this so to check your opinion.

The thing is simple like that: whenever I switch the UFX+ off (but so was with my previous UC)  before the iMac the connection with mouse and keyboard gets lost, resulting in having to reboot the iMac.
The problem happens with the bluetooth connection resulting unavailable, which seems reasonable. An electric problem maybe?
Interrupting the USB connection with the sound card is affecting the Bluetooth, that is what I know.
The Bluetooth icon on the upper right corner turns off and the mac shows the message of having lost connection with the mouse.

These are the specs:
iMac (Retina 5K, 27-inch, Late 2015) 4 GHz Intel Core i7 - 32 GB 1867 MHz DDR3 running macOS 10.14.6
Drivers and firmwares are the latest available, however these are the specs from the settings page:
TB: FW v109 Driver v1.12
USB: FW v54 Driver v3.222B