Radio Button Not Updating Save Value

I have a radio button that is defined as follows:

a!radioButtonField(
	label: "",
	labelPosition: ri!labelPosition,
	choiceLabels: {
		"kg",
		"lbs"
	},
	choiceValues: {
		1,
		0
	},
	value: ri!weightUnit,
	saveInto: {
		ri!weightUnit,
		a!save(
			local!BMI,
			rule!INS_RULE_BMICalculator(
				height: ri!height,
				weight: ri!weight,
				heightUnit: ri!heightUnit,
				weightUnit: ri!weightUnit
			)
		)
	},
	showWhen: not(
		ri!readOnly
	),
	choiceLayout: ri!choiceLayout
)

When I change the value from kg to lbs, the value of local!BMI is updated.  But, when I switch back to kg, it's not updated, when it should be.  I've tried testing the INS_RULE_BMICalculator with the values stored in the rule inputs, and it works fine. Does anyone have any ideas as to what I could do differently to get the local!BMI variable to update any time the value of this radio button field is changed?