When the device is set to use MTP, neither Windows, nor ADB will detect the device when connect via USB cable. I have tried multiple cables, in multiple USB ports. I have not tried other computers yet, as this is the only one that actually has the driver installed.
Disclaimer: I am running an Unofficial CyanogenMod 12 build of Android 5.0.2
Device : LG G3 (LG-D855)
Android 5.0.2 CM12-Unofficial-20141228
Google USB Driver: Installed
USB debugging: Enabled
PC: Windows 8.1 Pro Update 1
No one, except me, has reported this as an issue in the XDA thread.
I have been unable to get my PC to detect the device via USB since I first installed CM12, which was around 11/10/2014.
If i change to PTP, Windows will detect the device, as will ADB. But Windows will only let me access the internal storage, and Camera folders.