i think of two posibilities.
Its either the naming of your folders/files, because with your windows batch converter, which has resolve naming, it works. Or you checked the import setting that makes images being imported individually and not sequenced, i dont remember where that setting is. I believe its in the media window somewhere, you click the (...)
Ps, you should stay on 15.3.1 it is way faster and stabile compared to ver 16 with cdng. Also the layer mixer when applied to a timeline node does not work properly in the versions after 15.3.1 and reverting back to 15.3.1 does not fix it, once you brake it you lose it.