As is well established, the symmetry breaking by isotope substitution in the water molecule results in localisation of the vibrations along one of the two bonds in the ground state. In this study we find that this localisation may be broken in excited electronic states. Contrary to the ground state, the stretching vibrations of HDO are delocalised in the bound vertical bar 1a(1)(-1) 2b(2)(>)(1) core-excited state in spite of the mass difference between hydrogen and deuterium. The reason for this effect can be traced to the narrow canyon-like shape of the potential of the vertical bar 1a(1)(-1) 2b(2)(1 >) state along the symmetric stretching mode, which dominates over the localisation mass-difference effect. In contrast, the localisation of nuclear motion to one of the HDO bonds is preserved in the dissociative core-excited state vertical bar 1a(1)(-1) 4a(1)(1 >) . The dynamics of the delocalisation of nuclear motion in these core- excited states is studied using resonant inelastic X-ray scattering of the vibrationally excited HDO molecule. The results shed light on the process of a wave function collapse. After core-excitation into the vertical bar 1a(1)(-1) 2b(2)(1 >) state of HDO the initial wave packet collapses gradually, rather than instantaneously, to a single vibrational eigenstate.