Hi,
I'm afraid this is the expected behavior and there is no option available in Outlook to change it.
When we set a delegate, only meeting requests and responses to meeting requests will be sent to the delegate. If the recipient directly reply to the meeting decline message, the reply will be sent to your boss.
As a workaround, you can click the 'Direct Replies To' button under the Options tab when declining the meeting request.

Set the 'Have replies sent to' option to your email address, thus the replies will be directed to your mailbox.

Hope this helps.
Regards,
Steve Fan
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