Can expressions be used within an interpolator?

[jaybo]

mapbox-gl-js version: 3.1.2
mapbox-gl-draw version: 1.4.3

Steps to Trigger Behavior

Here's an example of an expression I would expect should work, but seems to not. No errors are shown, but the icon size doesn't change when selected.

// try and increase the size of the selected point by 2X
iconPrivateSizeInterpolator: mapboxgl.Expression =
[
    "let",
    "selectBoost",
    ["case", ["==", "active", "true"], 2, 1],

    [
        "interpolate",
        ["linear"],
        ["zoom"],
        0,
        0,
        4.5,
        ["*", ["var", "selectBoost"], 0.4],
        15,
        ["*", ["var", "selectBoost"], 0.7],
    ],
];

which is referenced like:

layout: {
    "icon-image": ["get", "user_icon"],
    ...
    "icon-size": this.iconPrivateSizeInterpolator,
},

What's totally curious, is that I have another interpolator based on a user property that works. But what I don't get is why I don't need to preface the variable name with user_ in functioning example below. Draw is sooo mysterious.

// WORKS boost size of text based on properties.starsAverage 
iconTextSizeInterpolator: mapboxgl.Expression =
// starsAverage is 0 to 5
[
    "let",
    "starBoost",
    ["*", ["coalesce", ["get", "starsAverage"], 0], 0.5], // 0 to 2.5

    [
        "interpolate",
        ["linear"],
        ["zoom"],
        0,
        0,
        11.5,
        0,
        12,
        ["+", ["var", "starBoost"], 11],
    ],
];

referenced as:

layout: {
    "icon-image": ["get", "icon"],
    ...
    "text-size": this.iconTextSizeInterpolator,
},

Expected Behavior

Icon gets twice as large when selected

Actual Behavior

No change in icon size