Although an electron or photon wave is meant to just diffuse and interfere, when it hits the screen in a certain diffraction pattern at the end, the particle version would be considered to have changed velocity and so lost/absorbed energy from/to the grating. Is this considered completely irrelevant?.
The diffraction pattern is determined by the particles that do not interact with the slit material. The ones that do are assumed be eliminated from the bundle. So you lose particle flux, not particle energy, and the precise physical interaction is not important.