Why encapsuled ultrasonic sensors have bigger dead zone than not encapsuled sensor?
Another reason why there is a difference in minimum distance is because the waterproof one has only 1 transducer (part that converts ultrasonic to electrical energy and vice versa).

So having only 1 transducer means that the same transducer has to be used for both transmitting and receiving keeping a minimum gap time in between to allow switching from transmitting from receiving mode.

HC SR04 has 2 transducers therefore there is no such limitation.

I was bugged by this limitation too.

I wanted a waterproof ultrasonic sensor with a small minimum distance and I was searching for one and I'm happy that I found out there is a new sensor.

https://www.ebay.com/itm/253978673298

It has 2 transducers instead of 1 so it has smaller minimum detection distance as compared to JSN SR04T.

Also its main board is not exposed like the JSN SR04T since it looks properly sealed and is therefore fully waterproof.