Indeed, with KP_0 and 0 only the last one works.
I don't know why, but putting the <primary>equal after <primary>plus do indeed work with both shortcuts working correctly. Even I am surprised that I did it.
I'm using the latest geany from the official arch repo.
Maybe the compiler optimized something that can make equal and plus working together and not 0 and KP-0?
I am at loss, but at least it's working with both shortcuts to zoom in for now c: