I'm designing a TNT cannon but I keep finding that the TNT destroys the blocks even in water.

In the original video 8 TNT are dropped simultaneously, and aligned so they appear to be fully submerged in the water.

I tried this test in older versions (1.12.2, 1.14.4, 1.15.2) and the same happens.

This only seems to happen with 3 or more TNT, and when the water block in which the TNT explodes is one block deep (though it can sometimes happen with deeper water) and bordered on at least one side by one of a certain group of blocks (see below). It is not consistent so sometimes no blocks are destroyed.

The group of blocks that cause this issue seems to be those which require a support block but are not destroyed by water, such as: Pressure plates, Signs, Scaffolding; but not (I haven't a clue why) Bells, Lanterns

I'm using vanilla singleplayer creative.