Select menu dropdown bug: optgroup and multiple selections

As noted in this post, adding optgroups to a <select> works if you add the raw HTML in the CF7 editor.

However, the combination of optgroups and multiple does not work correctly.

• Expected outcome: each of the multiple selections are sent in the email response
• Actual outcome: only the bottom selection is sent in the email response
• Test case: demonstration of the bug

https://ww.wp.xz.cn/plugins/contact-form-7/