Issue with barchart using traits

[seraf-m]

When using traits to make an interactive barchart, the 's' input to the barchart function does not update upon changing a trait in the ui; I have put together a minimal example I attach below. To reproduce, interchange the 's' and 'z' arguments on line 27.

At least on my machine, when that line (the else case in the **update function) spells
self.plot.mlab_source.trait_set(x=self.xs, y=self.ys, s=self.zs, z=radialsine(self.xs,self.ys,self.delta)),
the z values update nicely and the bars float to their specified positions, while the update to s is ignored (which is opposite of what im trying to do, I'd like to keep z at 0 and change the values of the bars).

I tried pretty much everything I could think of, changing order of arguments, trying the different number of arguments supplied to barchart etc., to no avail.
bartest.txt

I am new to reporting issues on github, if it would be more desireable to have the code in a .txt file, please let me know.
package versions:
mayavi==4.8.2
traits==6.4.3
traitsui==8.0.0
vtk==9.3.1
QtPy==2.4.1
PyQt5==5.15.11
PyQt5-Qt5==5.15.2
PyQt5_sip==12.15.0
pyface==8.0.0

import numpy as np
from mayavi import mlab
from traits.api import HasTraits, Range, Instance, on_trait_change
from traitsui.api import View, Item, Group
from mayavi.core.api import PipelineBase
from mayavi.core.ui.api import MayaviScene, SceneEditor, MlabSceneModel

def radialsine(x,y,delta=0):
   return np.sin(np.sqrt(x**2+y**2)+delta)

class bartest(HasTraits):
   delta = Range(-np.pi,np.pi,0.)
   scene = Instance(MlabSceneModel, ())
   plot = Instance(PipelineBase)
   xs,ys=np.mgrid[-2:2:10j,-2:2:10j]
   zs=np.zeros_like(xs)

   def __init__(self):
       HasTraits.__init__(self)
       self.scene.background, self.scene.foreground = (1,1,1), (0,0,0)

   @on_trait_change('delta, scene.activated')
   def update(self):
       if self.plot is None:     
           self.plot = self.scene.mlab.barchart(self.xs, self.ys, self.zs, radialsine(x=self.xs,y=self.ys,delta=self.delta), vmax=1,vmin=-1,lateral_scale=0.3,colormap='viridis',figure=self.scene.mayavi_scene)
       else:
           self.plot.mlab_source.trait_set(x=self.xs, y=self.ys, s=self.zs, z=radialsine(self.xs,self.ys,self.delta))

   view = View(Item('scene', editor=SceneEditor(scene_class=MayaviScene), height=250, width=300, show_label=False),
               Item('delta'))#, format_str='%.2f', editor_args={'low_label':'-π', 'high_label':' π'}))

my_model = bartest()
my_model.configure_traits()