I don't know whether this is helpful to anyone, but I experience the same/similar problems.
From what I can tell, the usb-c power inputs are not very robust, to put it mildly. I've a vast selection of 5 volt/1 ampere usb power supplies, only two of which will play nicely with the LA-QXD1 (I've two of them-the DACs, that is). Interestingly, I've more than one of both of the supplies that work (one Amazon and one Samsung), but only the two I'm currently using work. None of my powerboards with nominally 5V1A usb sockets will give them what they want (although I did get a spectacular ground loop). Both units took endless fiddling to "align" the usb power input cables so that contact wasn't intermittent. I'm in London, so very humid air at the moment, but I've had quite nasty little static shocks from both units. I got the same drop-outs with optical and usb as coaxial inputs (my actual use-case), and all could be coaxed (no pun intended) to proper functioning when holding the usb c power cable dead still and swearing under my breath.
I have no idea whether the units' pickiness has to do with how still the feeding supply keeps the cable, or whether these supplies are generally manufactured to a less exacting tolerance than the DACs require. "Common sense" suggests the former, but I'm pretty ignorant of this sort of thing.
I'm contemplating buying one of those "audio" power supplies like the topping P50 if I can find some short, stiff usb-c cables to keep everything in position to see if that helps. I shall post here if that works.
It's something of a shame (to me, at least), that the lion's share of the user manual was devoted to glowing test results, rather than the traditional "user manual" and troubleshooting parts.