Hello, there are 2 parameters that affect the animation of the mouth and the extension of the lips. Basically, scene creators use the KPPE settings on the timeline.
The parameters Base / Scale / Rotation cf_J_MouthBase_ty, Base / Scale / Rotation cf_J_MouthMove are responsible for them.
You can disable them on the timeline and reset the mouth parameters by selecting the character and go to KKPE and in the Bone section and reset the parameters.
These parameters are used in 90% of cases if the animation is without using the BetterPenetration plugin. Usually they use mods for different types of penises.
But sometimes even when using the BP plugin and the penis as a separate object, animation occurs, this is due to the settings of the character itself. you need to go to the male character settings in the BetterPenetration menu section and reduce 2 parameters to 0, namely MaxPull and PullRate.