Independent theta and radius in pie chart renders incorrectly

[nine9ths]

What happened?

I started with this chart:

import altair as alt
import pandas as pd

data = pd.DataFrame({
    'category': ['A', 'B', 'C', 'D'],
    'value': [40, 20, 30, 10],
    'radius_value': [400, 500, 400, 400]
})

chart = alt.Chart(data).mark_arc().encode(
    theta=alt.Theta(field="value", type="quantitative", stack=True),
    radius=alt.Radius(field="value", type="quantitative", scale=alt.Scale(zero=True)),
    color="category:N"
)

chart.show()

This draws as expected.
I then changed the radius field to "radius_value":

import altair as alt
import pandas as pd

data = pd.DataFrame({
    'category': ['A', 'B', 'C', 'D'],
    'value': [40, 20, 30, 10],
    'radius_value': [400, 500, 400, 400]
})

chart = alt.Chart(data).mark_arc().encode(
    theta=alt.Theta(field="value", type="quantitative", stack=True),
    radius=alt.Radius(field="radius_value", type="quantitative", scale=alt.Scale(zero=True)),
    color="category:N"
)

chart.show()

The stacking no longer applied correctly causing the wedge with a different radius to be rendered separately and on top of the other wedges.

In the compiled vega, the stack transform gains a groupby:["radius_value"] which if removed then renders as expected.

What would you like to happen instead?

I expected that by supplying the radius as a field I would be able to modify the radius without changing the stacking of the graph.

Which version of Altair are you using?

5.5.0