Currently TNT cannons with multiple projectiles are unpredictable, as of the 1.14 Snapshots and its First pre release it does not Follow rules it Previously has.

The projectiles of a TNT cannon are no longer exploding the way they should, instead of exploding at the same time and location they spread out over a long distance and if one might get lucky enough you could be able to hear or see the different Explosions.

We have been Testing it throughout the past few Snapshots and have yet to find a Solution to this Problem.

We are currently experimenting with Commandblocks to ensure that all the TNT is in the same Location, has the same delay and receives the same amount of pressure from the charges.

We even tried glitching the TNT into certain blocks, so that the TNT is unable to move a Pixel, but none of that worked or improved anything in the slightest.
We are aware that with certain techniques would allow one to create an "Invisible" Delay between multiple TNT's but we have, after additional testing, realised that the chances of this being the Case with Commandblocks are as close to impossible as it gets.

We believe that there could be some Problems with the current way collisions are Calculated/Created.

Here a few examples of TNT behaving wrong:

-TNT Exploding mid air and Stretching over a large distance

https://streamable.com/1vmsk

-TNT Splitting mid-air and Stretching :

https://streamable.com/i3nsl

-TNT Suddenly shooting down at an angle at the end of the TNT stretch: https://streamable.com/a2w41

All of these Clips have been created with the same Command-Block rig, without any changes to the Cannon.
We used this :

https://imgur.com/a/Wom1zVW

//Edit

This should be a proove that its unintended behaviour
https://streamable.com/7x6g9
each tnt entity is summoned by commandblocks, and should get effected the same.

Also the TNTs left and right should have cancelled each other out, shooting all