A cannon created facing south will be unable to aim left when facing south. If put in a turret, and rotated in a different direction, there is no problem. Only a cannon created facing south facing south.
Version [1.12.2]

I've noticed this bug still persists on moving crafts (movecraft plugin). It seems to occur when i rotate my ship and most often aiming south has the same issue. I've tested this quite a bit, here is a short video of an example. Regular cannons that are not rotated dont seem to experience this since your adjust which is good.
https://drive.google.com/file/d/1yH-72mAvkGPEbU48WsLox_l69yomcJo-/view?usp=sharing

In the cannon design config under angles you can find an entry which sets the direction of the cannon

defaultHorizontalFacing: west

Please try a different direction here and restart the plugin.

That seems to be a rounding issue if you go into negative numbers. I confirmed it on my server, should be a easy fix.
Thanks for the detailed bug report, the video helped a lot.

EDIT: The problem should be fixed

@DerPavlov

I think the original bug writer should have mentioned that it only happens when clock aiming. Not barrel-clicking aiming.

Here's a video of it happening.
https://drive.google.com/file/d/1SvtIEPfrxwTlAjPY7B28TUnV2zLNASeX/view?usp=sharing

I can confirm it happens on 1.12.2 and 1.15.2 so far.

I tried with every value in defaultHorizontalFacing: west too.
Setting it to North makes it affect aiming west.
Setting it to East makes it affect aiming north.
Setting it to South makes it affect aiming east.

That's great news @DerPavlov. Is there a place to download this fix? Bukkit, Spigot? Would really appreciate it. Thanks.