This is not an ST issue, but I'm hoping I can draw on the collective experience and wisdom of the community for this persistent and annoying issue in my overall setup.

My gigging setup is my ST4 iPad connected via USB cable to the USB input on my Behringer X32 Rack mixer. The X32 is primarily for running our IEMs (analog splitters all before the X32) but some of the ST4 multitrack audio we also route back out to FoH. Specifically on the X32, I have replaced the default X-USB card with the X-LIVE card (USB interface + 2 x SD Card slots for multitrack recording/playback).
From a ST4 perspective, our songs are generally:

Backing track (stereo) -> Backing Bus -> stereo input on mixer -> FoH and IEMs
Click and Cues (stereo) -> ClickCues Bus -> stereo input on mixer - > IEMs only

The Problem

Every now and then, in the 30-90 mins between end of soundcheck and start of first set, the USB input just seems to die/go to sleep. In that period, there is no audio going through the X32 mixer or USB interface - it is just sitting there quietly on stage waiting. When the issue occurs, the rest of the X32 mixer is all working fine, but I go to hit play on ST4 to start the set and there is no audio from ST4. ST4 can see the interface and thinks everything is fine, the song is playing, but there is no audio coming from the USB interface in the mixer. Power off the X32, power it back on, and we're all working perfectly again. I have tried plugging/unplugging USB cables in every order imaginable, changing USB cables, changing adapters, etc. but power cycling the X32 is the only way to resolve this scenario.

Mitigation Strategies

Of course, I can't rule out faulty hardware, but I don't have another X-LIVE card to test and they aren't the easiest to find in my part of the world without simply ordering one. They aren't that expensive so that is an option.

However, given we've never had this problem once we start playing, across 15-20min set breaks, for 3-5 hours from the Start of Set 1 to end of gig, I'm hoping that if I simply keep some sort of audio signal being fed from my ST iPad to the X32, that will prevent the issue from occurring.

Now because I don't know if my theory is accurate, I've decided that while I could just route some audio into an X32 interface input that isn't routed anywhere, I have no way of knowing whether this solves the problem or not until the problem re-occurs. So initially, what I think makes more sense, is to route some audio into my ClickCues Bus that never routes anywhere except our IEMs (i.e. no chance it could accidently go to FoH) and that will allow me to just turn on my IEM receiver pack any time pre-show and just check "can I hear audio" and have confidence the interface is alive and working.

Continuing down that line of thinking, I came up with the following options.

1. Use the Failover signal
ST4 has a built-in Failover test tone feature (https://stagetraxx.com/docs/v4/settings/audio/audio-settings/#failover-signal). While it's primary purpose is to be fed into hardware that monitors for the tone, there's no reason I couldn't route it into my ClickCues Bus and manually turn it on at end of soundcheck, and then turn it off just before we start Set 1.
Downsides:

I have to dive into Settings on the Host ST4 iPad to manually turn off
It sounds awful in IEMs if you turn them on before I turn the tone off 🙂

2. Create a special pre-show song
I could create a song in ST4 to put at the top of our setlist that just has a 2 hour audio track of a shaker pattern or similar. I route the track into the ClickCues Bus and turn the volume down low so that it doesn't perforate an eardrum like the failover tone 🙂
At the end of soundcheck, I start the song and just let it run for however long the gap is between end of soundcheck and Set 1 start.
As we prepare to start Set 1, I stop the song and select the first song we plan to play, and we're good to go.

3. Use any other audio player
Of course, I could use any audio player on the iPad to play audio, as long as it lets me choose which interface input to route the audio to. I could then just run this in the foreground after soundcheck, then kill it when we're ready to start Set 1.
The only downside to this is that I keep my ST4 iPad as stripped down as possible with no unnecessary apps, so this just means having another app. Not a big deal.
I'd need to give it audio content to play (can't use a streaming app as the iPad is connected to our closed band network... unless I connect it to a hotspot, etc. which just introduces more things I have to remember to do/change to get back into gig configuration so definitely not desirable), or use an app that can just play a short audio sample on loop, etc.

At this stage Option 2. is my preferred and what I can easily setup.
Are there any other strategies people are using successfully that I should consider?