So this is FL Studio bug

[lapis phoenix]

I don't know how and why, but if you put the panning in the FL Studio Mixer, it will turn your audio signal to mono
So my friend just testing it by route the audio signal (stereo) to 2 other bus, 1 bus panning full left, 1 bus panning full right and guess what happend? Yeah, the Vu Meter showing at the master bus is mono, channel L and R is the same.
You can try it out and don't forget to notice Image-Line about this. Thanks!
Btw im not native speaker so my English such, so if you feel uncomfortable just skip this.

Btw shout-out for Blue Harizon and Xuan Tung Hi-end for found this as seen in video
https://www.youtube.com/watch?v=vWHFRhi8Q0Y

[obsidian iron]

This is because of the circular pannning law of the mixer. If you want to split the level with no cross channel mixing you need to use the Fruity Stereo Shaper

taken from the manual https://www.image-line.com/fl-studio-learning/fl-studio-online-manual/html/mixer.htm

[lapis phoenix]

Thanks for letting me know, I think if Image-Line make a option to choose panning law it will be better

[sick cairn]

they have that option

you can pick between circular or triangular

triangular means the daw doesn’t compensate for audio change when you pan