I have notice very same thing. I've found the mixing in FL Studio is very difficult. I mean, no matter what you do, you can't get that sound what you want. It is endless tuning. Then, rarely when you are "okay" with mixing, as soon you export it, it sounds so ****. Lifeless, no dynamics. Cold. Nothing like how it sounds inside in FL. It simply sounds so different, so bad.
I know that mixer resamping should help you hear things more realistic way, but it just doesn't working. I.E, If you have mixer resamping 128-point sinc, you should render your final mix on 128-point sinc too! The thing is, that it still doesn't sound same. I have good soundcard, I know all the things and settings, and I have tried everything. Nothing helps. I have give up. For these reason, I have real difficulties to finish any projects in FL. Currently, I have +20 unfinished projects in FL.
Then, when I do song like in cubase, I make it long long time, due different (difficult) workflow, but the mixing is "super easy". I get that sound what I want really quickly, and it sounds really good. Rendered file sounds exactly same is in cubase. No problem at all.
I have tried to move away from FL Studio. Only problem is, I just can't. I have try it so many times. Really. Around 2 years, I have "moving" away from FL. Every time I come back in FL. Cubase could be nice choice for new host, but I don't like cubase workflow. Not piano roll, not automation system. Not zooming and many other things. FL has so unique, best and fastest workflow. Piano roll is like 100x better than in any other host. FL's workflow is like made for me, that's why I have real difficulties to leave FL Studio.
This is endless loop. I don't want to start another project in FL, cause I know, that I will don't finish it anyway. I don't want to start make song in cubase or any other host, cause I can't "fulfill" my ideas, my style in other host's. If I try to make song in cubase, it feels like I have never ever make music before. I am so lost, feeling so empty. This leads my story here, where I haven't do full song in 8 months. I am really lost and I don't know what to do.
If you go to whine about sound quality in IL forums, they all attack you blaablaa. They always paste those **** audio myth youtube links blaablaaa. I am biggest FL fan here. I have been use it so so many years, but the thing is, there is something wrong in FL sound engine for sure. I am surprised how others can't hear or notice it.
sry for my bad english..