What discovery supported Mendeleev's decision not to place elements in order of relative atomic mass

There are two things to consider here. At the time of producing his periodic table, not every element had been discovered. Mendeleev recognised that there must be missing elements based on the properties of the ones that had, so left gaps for undiscovered elements which were found later - scandium, gallium and germanium for example. Tellurium (52) and iodine (53) would also have been in the wrong places if organised by their atomic weights, so these too were rearranged.