Sorry for the necro-post but this is an issue I have dealt with more than once. It just currently cropped up again just this week after receiving two new JLF's.
I also noticed the original poster did not follow-up as to whether replacing the springs and actuators helped. I wanted to add my experience with this issue to the post.
While it is a debounce issue, the problem is often the omni switches themselves in the JLF's, often these switches are excessively noisy. Fix is replacement of the affected switches or better yet replace the entire switch board assembly with a more robust solution. (example: Cherry mod).
I received 2 new JLF's this week and one stick has two noisy switches. One switch so bad it made navigation impossible in one direction. Swapped out the switch plate and worked perfect, lower noise, no extra inputs.
Noise is very visible on an ocilloscope but all the switches look noisy so it is difficult to see the threshold where controller translates to an input.
Setup as tested:
2 control panel mockups with I-PAC 2's.
I-PAC 2 firmware 1.44, Switch de-bounce set to Short
Multiple Sanwa JLF-TP-8's, stock springs and actuators, octagon gates. (purchased from reputable online Arcade Shop, not Amazon, be wary of JLF clones).
Test panels running RetroPie 4.4 and Emulation Station 4.7.5. Additional tests on a PC running Python key capture script to log key inputs.
In the bartops I have built (primarily with Sanwa sticks) I had several sticks with ghost inputs, often very bad in one direction, better in the opposite direction. Fine in the others.
Replacing the switches or entire plate has always fixed the problem for me. Another note is that the plate does have a fair amount of flex/movement when joystick is in full deflection, I don't know whether this contributes to the noise, but I have not had ghost issues with modified sticks that remove the switch plate entirely.
I stumbled upon this post and its recommendation to adjust software debounce, which should not be needed if the switches are good.
Hope this may help the next person,
-disgusdad