Condition isn't triggered on default value

I have a select list field as dependee (field_a with options A, B, and C; value A is default) and a text field as dependent (field_b). I want field_b to be visible only when field_a has value B. That does work when I change the value of field_a in the node edit form, but it doesn't when the node edit form is loaded (with default value A for field_a).

To be clear, expected behaviour after loading the node edit form would be:

• field_a has value A and is visible
• field_b is hidden

But it is:

• field_a has value A and is visible
• field_b is visible