Lenovo Zuk Z2, Z2 Pro, Z2 Plus phones all have a bug inside the ROM - if an app asks Android to show the Settings screen to let the user grant permissions to display a floating window, the ROM refuses to do so.
During a call Boldbeast Recorder needs to display the record button as a floating window over the Dialler app, because of the bug above in the ROM Boldbeast Recorder can't achieve it. So you should manually enable the "Settings - Advanced Settings - Applications - Boldbeast Recorder - Permissions - Display Floating Window" option in your phone, then Boldbeast Recorder works at once.