i cant really help you with the solution to the problem, but i tested a bit more:
first i installed pfsense 2.4.2 on an old computer with complete different hardware and i had the same problems
then i installed pfsense 2.3.5 (32bit) on that old computer, and voila: it worked.
After a restart it didnt work and i always had to reassign the interfaces again. using the tricks to avoid that didnt help, since the interface itself was found on boot but wasnt available as ue0.
the only way to get the interface to work after a restart: when the reassign-dialog came up, unplug the usb-interface and plug it in again. then just assign whatever interface you want and in the end, where you have to answer the question if you want to continue with that settings, just say no. and then it uses the re-plugged interface correct.
after all the testing i think the interface or the interface-driver itself is not the problem. i think its the usb-driver itself or maybe some energy-saving-stuff inside it.
i gave up after that since i dont have many options to change energy-saving in bios.
maybe you can try and disable all energy-saving-stuff in bios that could interfere with usb.