If you uncomment the code on line 15, it works properly

kv_eval_problem.py

from kivy.app import App
from kivy.app import Builder

from kivy.uix.stacklayout import StackLayout
from kivy.uix.label import Label
from kivy.uix.button import Button

from kivy.properties import NumericProperty

Builder.load_string("""
<MyLabel>:
    size_hint: 1, None
    font_size: app.some_multiplier*10
<MySubLabel>:
    font_size: 10 # or app.some_multiplier
""")

class MyLabel(Label):
    pass

class MySubLabel(MyLabel):
    pass

class Test(App):

    some_multiplier = NumericProperty(1.0)

    def build(self):
        root_layout = StackLayout()

        root_layout.add_widget(MyLabel(text="MYLABEL"))
        root_layout.add_widget(MySubLabel(text="MYSUBLABEL"))

        property_change_button = Button(text="Change Multiplier", size_hint_y=None)
        property_change_button.bind(on_press=self.change_multiplier)
        root_layout.add_widget(property_change_button)

        return root_layout

    def change_multiplier(self, *args):
        self.some_multiplier += 0.5

if __name__ == "__main__":
    Test().run()

While using 'or' is a cool trick, kivy should always respect the user defined rules no matter what.