• On a fresh eZPlatform V2, create a new content type with a ezselection field
• Add 2 options (A and B) in the ezselection field

• Create a new content object of this type
• In the ezselection field, option A is visually selected. Don't do anything on the ezselection field
• Publish the content
• Value A is not sent in the POST request

• Edit the object, select option A in the ezselection field
• Publish the content
• Value A is not sent in the POST request

• Edit the object, select option B in the ezselection field
• Publish the content
• Value B is now sent in the POST request

So, it looks like there is a Javascript bug on the select dropdown plugin.
Also, if the field is not required, it should have an empty option.

My advice (do what you want with it...) : drop that ugly plugin and keep the native select dropdowns.