LG D850 does not show on PC using MTP - PA 4.6
I have experienced this issue myself and it was because when you plug the phone in the first time, windows automatically tries to locate drivers and by default uses the Google Nexus ADB Interface.

If you go to the LG website support section you can specifically download the LG G3 drivers and install those instead. I would recommend removing the existing entry from the device list first (phone unplugged) and then running the LG drivers and plugging your phone back in afterwards.

This fixed the issue for me and now whichever option (MTP, PTP and debug mode) all work properly.