I know just about every monk with this skin would really appreciate it to be visable while sheathed. We all invested time to get it and it would really make it feel more special to be visible. 
If you mean Xuen’s Enforcer fist weapons. yea no thanks. Those things are massive and it would be a huge eyesore having them sticking off of your hips. Sheathing on the back would look just as goofy.
At least give me a choice
The issue of fist weapons being invisible when sheathed was raised when the WW artifact was first announced. We were told that they were working on a solution for that and went radio silent on it. We kept asking for months during Legion’s development for some sort of update - which we finally got a few weeks before pre-patch: They weren’t going to do a gorram thing about it.
I’m not still upset at that¡ Honest¡ 
Windwalker artifact should have been having a corrupt/evil celestial that was banished in Pandaria’s early history merge its spirit with you to give you the strength to fight the demons in exchange for its freedom. The whole journey through legion could have had great dialogue between the monk and the celestials spirit from within that put the celestial on its path to redemption leading up to it willfully sacrificing itself to purge the corruption from Sargaras’ sword poisoning Azeroth.
The actual weapon could have been effects placed on the monks fists, fire effects, wind effects, lightning, glowing, etc. Something that symbolized the monk’s fist as the weapon.
But no we got pizza cutters.