I have experienced a very similar problem, on FW6, USB Audio Driver 5.30, 5.58 & 5.68p, detailed here:
https://forum.fractalaudio.com/threads/usb-audio-driver-update.202612/
To summarise I was getting nothing out of Output 1 on the unit, but if I switched the XLRs to Output 2 I got a continuous loud, "overloaded" noisy signal, which would be anything that would normally be played out through the unit output(s) but just incredibly loud and distorted. If you read the other post you'll get more of an insight into exactly what was happening. It's only happened once again since I raised the support ticket, and on that occasion Output 2 wasn't even in the Preset that was loaded, yet it was still producing this incredibly loud, noisy signal whilst Output 1 and Headphones were doing nothing. In my case the signal isn't getting "lost" in the chain anywhere, because Output 2 isn't in the chain, nor has it ever been. Something is getting screwed in the internal routing at switch on for whatever reason.
My initial thought was that it was a USB related problem because in my case I only started seeing it after updating the USB driver to the latest version. Uninstalling the USB driver and re-installing the previous version(s), didn't fix it. Now that I've read something similar has happened to people with devices running FW5 as well, I'm starting to wonder if this is a problem that has been around for a while, but which is only triggered by a specific set of (fairly rare) circumstances.
I can fix it with a reboot, but it puts me on edge, wondering why it's happening and when it's going to happen next. I'm now actually hoping it will happen again soon, so that I can help to resolve it