Why was Rishabh Pant denied four runs despite ball racing towards boundary?

The incident happened in 40th over when it seemed as if the ball hit Rishabh Pant's pad and sneaked past the leg-stump for a four. As the batsman had opted for DRS, the on-field decision was overturned. The viewers were perplexed to see the umpiring not signalling a four on that occasion. The rule states that, if a batsman is given out leg before, the ball at that moment becomes dead even if the decision is overturned later.