When using command blocks to replace lit TNT with new TNT, placing fire on top of the TNT moves it one block below

How to reproduce 

Place two command blocks next to each other 

First command block : repeat , alwaysactive 0 ticks delay 

Command for the first command block : execute @e[type=tnt] ~~ setblock tnt 

Second command block : chain conditional always active 

Command for the second command block : kill @e[type=tnt]