Select list widget ignores user input, uses default value instead

I encountered this with a taxonomy term reference field. To reproduce:

Make a taxonomy with some terms. In my case taxonomy "privacy", terms are "public" and "admin only".

Add taxonomy term reference field to a content type. Select that taxonomy.

Make the field optional (not required).

Select a default value (I chose "public").

Number of value: unlimited (multiselect).

Create an instance of the configured content type.

For your term reference field, select "- None -". Or, unselect all options. Either way reproduces the problem.

Save your new node.

Notice that your default value ("public") was saved, even though you had selected none!

Perhaps worth noting: if you edit the existing node, you can save it with no terms. That is, the problem happens when a new entity is saved, not when an existing entity is updated.

I'd be surprised if I'm the first to report a problem this serious. I googled but couldn't find it described elsewhere.