Version of ClickThrough 1.20.2-fabric0.89.1-0.4.1
It's impossible to edit a sign when it's on a trunk, even by pressing "shift". It is still possible to color or add glow to signs. If the sign is not on a trunk, it can be edited without any problem.

I'm in 1.20.4, using 1.20.4-fabric0.91.2-0.4.1, and I cannot edit signs at all by shift-right-clicking.
This is for wall signs, be it attached to a chest or a block. Once placed, right-clicking will click through, but shift-right-clicking does nothing.

If you're on 1.20.4, please use the 1.20.4 version of the mod. And if shift-clicking still doesn't work, you can always toggle the mod off and on.