I Created a small container out of Bedrock for TNT, covered from all sides, and activated it using a redstone torch. However, blocks were blown up outside the container for the TNT.
Linked issues
is duplicated by 6
MC-127360
Things surrounded by bedrock can be destroyed by exploding TNT
Resolved
MC-137832
TNT damages blocks beyond obsidian
Resolved
MC-192235
Explosion passing?
Resolved
MC-233780
Creeper's explosion damage goes through blocks, even in completely closed area.
Resolved
MC-240153
TNT can explode blocks even when completely covered in obsidian.
Resolved
1 more links
Thats because the blast from Tnt is a set radius including in the corners. since you don't have bedrock in those corners the blast technically isn't being contained since it has to start 1 block out from the tnt block in all dirctions (up, down, left, right, and diagonally).
See attached screen shot... the tnt is where the blast radius starts