Provided the attached video link works properly to showcase the issue, the fill command does not fill the provided area. The command only fills the top layer of the coordinate instead of a full box like it's supposed to.
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In the provided video, the coordinates in the command are to fill 1 10x10 area of TNT that is 1 thick at y-layer 12, which is what happens when the command is run.
In the provided video, the coordinates in the command are to fill 1 10x10 area of TNT that is 1 thick at y-layer 12, which is what happens when the command is run.
You're not specifying a box, you are specifying a single layer. Instead of:
/fill -9 12 -17 -18 12 -26 tnt
try
/fill -9 12 -17 -18 22 -26 tnt