Without checking the files I would guess that function A hook is working because Vuhdo is actually calling VUHDO_determineDebuff from outside file VuhDoDebuffs.lua and in those files the local version does not exist.
On the other hand VUHDO_customizeText is being called from within the files VuhDoBarCustomizerHealth.lua where local version is called instead of the global version. When you hook a function you in fact make another function but the local version is pointing to a pre hook version.
You can confirm with this macro
Code:
/run function a()print("function")end local f=a hooksecurefunc("a",function()print("hook")end) local g=a f()g()
f() call does NOT call the hook function because it is poiting to a version of the global function a that has no hooks
whereas g() call does call the hook function because it is a local reference to the hooked global function a.