Can't answer from my personal experience. Since I didn't relapse. But from I've seen from others journey. Yes, if you relapse during a flatline. It gets reset. That's the worst thing you can do during a flatline. Your brain is simply doing self msinstainence during the flatline. You will feel better after coming out clean. I no longer have mood swings after my flatline is over.
The answers are really everywhere. Look up "nofap flatline" on YouTube. You can find out a lot about the flatline. What is it and how to deal with it