Questions:
Is it a NOS cable or a modern "will fit" part? It's worth noting that F15 is listed as using *C101W-2275 cables and F15A *C01Q-2275A in case you ended up with the "wrong" part. I don't know the difference between the two cables. LWD parts show a cable tagged with the "correct" part number
https://www.lwdparts.com/product/cable-handbrake-f15/ They also list the F15A cable and might be willing to measure length differences (sheath and overall) if asked.
Is the cable fully seating in the holding bracket on the backing plate? (and at the other end?)
Would it be better to take some (but less) off both ends of the sheath? You see less rust than I do but it might prevent formation of a rust ridge in the backing plate bracket that could block future removal.