Owl feathers are for the Homing Tomahawk. Not the Improved Tomahawk.
I have the same issue. And it’s the only item left for me to craft to compete the Survivalist 7 challenge l. I actually had one in my inventory when I finished the Survialist 6 challenge and received the Survivalist 7 challenge. But it wouldn’t let me craft another one. So I got rid of it, thinking that might trigger it to become craft able again but it didn’t. I’ve re-read the pamphlet just in case, I have all the ingredients to craft an Improved Tomahawk and even Homing Tomahawks, I just can’t craft one again. In my inventory I have 3 regular tomahawks (0 Improved, 0 Homing), 25 eagle feathers, 8 hawk feathers, and 4 owl feathers and there’s just no option to craft an Improved Tomahawk or a Homing Tomahawk. It’s frustrating. I’ve given up. Hopefully you somehow get it completed. Best of luck.