Brake wiring question

The brake wiring could be inside the axle tube, or the wiring could be cut like you're suspecting. An easy test would be to hitch the trailer up and connect the umbilical. Then have someone slide the manual brake lever to full brakes. Put your ear by the suspect hub and see if you hear a hum from the brake magnet. If you hear the hum the wiring is hooked up, if you don't hear anything then you'll have to pull the wheel and hub.

Factory brake wiring is often not very good. Wire size too small, connector type bad choice, wires rubbing against metal, etc. If you do need to fix anything, you might consider making improvements rather than just "patch" it back like it was. If you've got a single axle trailer, having only one brake remaining is not good!

Proper operation of brakes on a trailer are extremely important. As mentioned, lift a wheel and see what happens when brakes are applied (pull breakaway pin but re-install right away when done). Manually adjust the brakes through the backing plate as needed. Also adjust the brake controller as per manual.

Actually, if it were me, I'd pull a drum off to see how the brakes look inside. It's so simple to do providing you can jack one wheel up at a time by putting a jack under the U-bolts. (Don't do that without a wood block under the U-bolts tho.)

Jack up trimmed side. Spin wheel, apply brake, knowledge gained

Most times the brake wiring is routed through the axel from one side to the other.

It's look like intentionally trimmed back.
Now I wonder if the right side is routed through the axle to the left side and I just can't see it....

The brakes do work on both sides to best of my knowledge.

No. Sounds like a broken wire or someone did not finish the job