The effect of diffraction on the conservation of the quantum state of a quadrature-squeezed light pulse in an ensemble of thermal atoms for a collinear configuration of light fields has been considered. The motion of atoms leads to the fact that when both the signal and control fields propagate in the same direction as when writing, there is an uncompensated phase incursion even in the case of forward reading. This phase incursion leads to the fact that the squeezed quadrature of the light pulse is mixed with a stretched one. As a result, the pulse squeezing can decrease significantly. The analyses of the effect of diffraction for different configurations of the multimode resonant quantum memory protocol has been performed and the ways to reduce this effect to a possible minimum have been found.