Very weird issue, since there are additional ( corrective ) joints surrounding the thigh area ( and also other areas, such as shoulders, elbow and wrist ) that are completely driven by the joint orientation using the RBF node ( Pose Driver ).
Does the pose look ok using Control Rig?
It might be that, for some reason, the additional joints are being animated froom the control rig, and this is what is causing the issue, so that a look at the corrective joints causing the bulge issue and check if there is an animation curve on it, and in case delete it, since, as mentioned above, the correctives are driven in realtime by the PostProcess AnimBP.