You can create invisible TNT entities by specifying water or lava

You can specify any block state desired for minecraft:tnt to make it render as that block. Water and lava are among the possibilities, but in those cases, TNT will not render anything besides its particles, when in reality you’d expect to see water or lava there.

To reproduce:

/summon minecraft:tnt ~ ~ ~ {block_state:{Name:"minecraft:water"}}
/summon minecraft:tnt ~ ~ ~ {block_state:{Name:"minecraft:lava"}}

Expected results: TNT that looks like water or lava.

Actual results: TNT whose only indications of existence are particles, shadows and subsequent effects.