When does dirtyState change?

[s-ohnishi]

I tried rewriting the field values as follows using the database of the sample application Invo.

$mdl = Products::findFirst();
$mdl->name = 'Cirsium';
$mdl->save();

DirtyState is 0 (DIRTY_STATE_PERSISTENT) at any timing and there is no change.
By the way, Products is in initialize ()

$this->useDynamicUpdate(true);

is running.
Also, when $mdl->name = 'Cirsium';, both getChangedFields() and getChangedFields() return array('name').

I expected dirtyState to be 1 (DIRTY_STATE_TRANSIENT) at $mdl->name = 'Cirsium';.

Looking at Model.zep, __SET(string property, value) doesn't seem to change dirtyState for a simple set of values.
Is it okay to change dirtyState only when the value is an array or a relation with another model is set?
Is it wrong to expect dirtyState to change when rewriting a field?