In creative mode, we should always break the blocks when left clicked them. However, a TNT block unstable=true blockstate is ignited.
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While this was the behavior with punch to ignite tnt in 1.12.2 and prior, I've gone ahead and change the behavior for when you're in creative mode as to reduce accidental destructive actions with unstable tnt. (And all the map makers rejoiced!) 😃
this is intended behavior. this was in older versions (tnt placed with a data value of 1 in versions prior to 1.13), and is going to be reintroduced in 1.13.1