The wired connection is not configured for 802.1x therefor a Machine Auth is not sent to Clearpass (there is no device to send it).
Machine Auths are only sent at the following times...
Machine Boot - Before logon
Logoff or Reboot
Also, this is only an issue if the user has not performed a machine auth during the period that is set for the machine auth cache in clearpass. IE.. if the machine auth cache is set to a week, they can stay docked on the wired connection for say 6 days, undock and easily transition to wireless since the Machine Auth is still cached. The machine cache timer in clearpass is also reset/refreshed every time the machine does a new machine auth.
So some people will configure the machine auth in clearpass to last for several days. So a user reboot (while on wireless) would usually resolve the situation. Then, most likely, they will boot up/shut down sometime within a week which in turn renews the cache in Clearpass.