My lg g6 detects monster.. I purchased a new charger and patch cord ....still the same . I use my wife's charger (same phone ) I works fine .I replaced charger and patch cord ..no difference
You may have some debris in your usb port that is conductive enough to trigger this. Try blowing it out with some compressed air, and if that doesn't help, you may need something to physically insert in there and clean it out. Trying to avoid damage, I've done this with the pointy end of a small zip-tie and it worked well. Soft enough not to damage anything by accident but stiff enough to push the lint out.
Mine started doing this after the most recent Sprint security update. May have been a coincidence. Either way, I tried cleaning out the port with compressed air, a toothbrush, and finally some contact cleaner (with the phone turned off). One or all of those did the trick.