<md-checkbox> indeterminate state cannot be applied after the element is attached

[SvetlozarValchev]

It seems that the checkbox's indeterminate state (which corresponds to value=null) is not available once the element is attached. You can initially set it to indeterminate, but you cannot set to indeterminate state with value changes.
From the code here, you can see that the value is checked only on attached, and it cannot be set to indeterminate on subsequent value changes:

aurelia-materialize-bridge/src/checkbox/checkbox.ts

Lines 54 to 58 in 2fa3313

	if (this.checked === null) {
	this.checkbox.indeterminate = true;
	} else {
	this.checkbox.indeterminate = false;
	}