@K4cus Is this a flybarred heli? I assume so since you have the H_FLYBAR option set. Here is what I propose to fix your problem. What I think is happening is that when you go to acro, since the H_FLYBAR is set true then the code is passing through Pitch, Roll and Yaw commands and that is why you are seeing the drift in the tail because it is not using the same flight control code as in stabilize.
So Set H_FLYBAR to zero. Then set the ATC_RAT_XXX PID values to zero for all three axes. Set the ATC_RAT_XXX_VFF params to get the control deflection you expect in each of the axes. Probably best to do this in acro mode. Doing it on the bench disarmed should be fine. This is typically between 0.1 and 0.2 maybe a little higher for a flybarred heli. Keep the ATC_ANG_XXX_P gains as the default values of 4.5.
Once last word of caution. Make sure you disable the crash detection. It looks for anytime the requested angle is off more than 30 deg from the actual angle and then disarms the aircraft. For heli’s this doesn’t matter because a crash is spectacular whether you disarm right away or not. The parameter is FS_CRASH_CHECK
What I’ve suggested above should give you what you want.