Hi,
It looks like a Mesh and Texture fix for Daedric bows back in 2016 has not to date been carried over and implemented for the Bound bow / Mystic Bound Bow.
Essentially, Daedric bows had an alignment issue, so were not gripped properly by the player’s hand. This was fixed in USSEP v3.0.1 (2016-01-30), but it seems that this alignment issue was not fixed for the bound bow, which shared this issue (probably due to their model similarity). From AFK mods:
v3.0.1 (2016-01-30)
Mesh and Texture Fixes
Daedric bows were not aligned properly to be gripped by the player's hand. (meshes\weapons\daedric\daedricbowskinned.nif) (Bug #19645)
Any updates on this would be appreciated. Thanks!