Doesn't Qubes 3.2. have access to kernel 4.14? If true, then this "might" be a possible reason. Qubes 4 has access to kernel 4.14. Currently Qubes 4 has received some qubes-usb-proxy updates, if these updates were only considered for Qubes 4, and not Qubes 3.2., then it might be an overlook pr. design? It's a lot of things to keep track of afterall, if I'm not mistaken, human error could easily happen here.
In which case, this update wasnt designed for Qubes 3.2. which doesn't have 4.14 kernel yet? But I'm a bit puzzled, shouldn't 4.14 be vailaboe for Qubes 3.2. as well? Considering meltdown, spectra, and eveything is fixed in 4.14. I was under the impression the reason for the unexpected sudden hasteful release of 4.14. was to fix spectra and meltdown.
Perhaps this is similar to the other Linux distributions that stopped working due to hasteful quick updates in a race to fix this issue? Only in Qubes might have been a less harmful mistake (if it was a mistake, I'm unable to confirm).
In other words, it might just be a question of reversing the Qubes Proxy USB back to the older one, and all the dependencies that follow?