I tracked the memory leak down to TransformState::make_pos_quat_scale, which is used to keep Bulleet & Panda3D in sync.
Can you confirm that the following script shows a fast increase in memory consumption:
from panda3d.core import Vec3
from panda3d.core import Point3
from panda3d.core import Quat
from panda3d.core import TransformState
q = Quat.identQuat()
s = Vec3(1,1,1)
i = 0
while True:
i += 1
p = Point3(0,0,i*0.001)
ts = TransformState.makePosQuatScale(p,q,s)