I also had a similar issue when building a GUI with several successive steps, with each step requiring showing a new slider and deleting the old one.
In the end I resorted to a hacky workaround:
function hide_slider(s)
s.blockscene.visible[] = false
s.width[] = s.height[] = Fixed(0) # If not, the hidden slider seems to grab the mouse interaction instead of the visible one
end