Just closed a case with TAC. The issue is when the Calling Settings is set to Auto-minimize calls on the When using a desk phone or video device for call: you lose the option to control what you are able to do with the call you are on. It shows you can transfer but only gives a grayed-out resume button with no option to complete the transfer.
If you select On All Calls it allows you to complete the transfer within the Webex call window with a green button. Not all users want the call window always popping up in front of them but still want the ability to open the minimized call window and be able to control the call.