This would be very bad for PvP if this is a feature. But it most likely is a bug. You can't place TNT on TNT.
MC-64794
Can't place TNT on TNT without shift clicking
Resolved
Duplicate
0
Duplicate of MC-64425 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.