visible_when on Label doesn't implicitly take the boolean of the specified trait

[nicolasap-dm]

Consider this code:

from traits.api import HasTraits, Str
from traitsui.api import View, Item, Label, UItem

class Foo(HasTraits):

    a_string = Str()
    fixed_string = Str("a_string is true (according to UItem)")

    traits_view = View(
        Item("a_string"),
        Label("a_string is true (according to Label)", visible_when="a_string"),
        UItem("fixed_string", style="readonly", visible_when="a_string"),
        Label("len(a_string) is nonzero (according to Label)", visible_when="len(a_string)"),
        width=420, height=200
    )

Foo().configure_traits()

When a_string is the empty string (therefore False):

• the Label(visible_when="a_string") does not get hidden
• for comparison, the UItem with the same visible_when, and the Label(visible_when="len(a_string)"), correctly do get hidden

Tried on main with pyside2.