Rear derailleur does not downshift properly, shifts up smoothly
This looks like a classic case of the cable sticking in the housing momentarily as the shifter lets out cable to downshift.

Your indexing looks fine as the chain is not rattling or trying to shift on it's own. FYI the derailleur high and low limits do not affect shifting or indexing.

Try removing the cable, wiping it down with a clean rag, applying a little chain lube and reinstalling. If that does not work replace the cable and housing. One cable and a length of housing is not expensive.