Velocity macros leak between consecutive renderings of the same page

Consider this example:

First, edit a page A and enter the following:

#macro(A $x)
  #set($x = "'a'")
#end

#A($x)
$x

As expected, saving and viewing the page displays 'a'.

Then, edit the page and delete the definition of the macro, leaving just:

#A($x)
$x

Saving and viewing the page, instead of showing the code unparsed, displays again 'a', as if the macro definition was still available.