If a TNT canon with 4 or more TNT's (facing any direction) it will blow up the cannon, even if water is used.
Linked issues
duplicates 1
Attachments
Comments 4
This has been fixed as MC-178630
MC-171977
TNT Cannons blow up if using 4 or more TNT's
Resolved
Duplicate
2
(Unassigned)
Community Consensus
Important
Redstone
Explosions, TNT
20w06a, 20w16a
Retrieved
Also found this issue. More specifically, it can occur when any 2 pieces of TNT overlap inside a water block (flowing or source) that has no block above it.
Placing a block above the the water before the TNT is dispensed prevents the issue from happening.
The cause appears to be to be TNT entities moving ever so slightly upwards when overlapped, but that's just a guess.