Driver Left Tail Lights Bulb Socket Panel & Plug Connector & Three Bulbs 63217313043 Compatible with BMW 3-Series 320i 328i 335i 328d 2012 2013 2014 2015 Rear Light Lamp Repair Board Kit

This is a known issue. It happened to my 2014. The ground pin of the tail light bulb holder assembly overheats and eventually loses continuity. For a while it'll be intermittent while you're driving and hard to replicate. There are a couple ways to solve it but I choose to replace the assembly and splice in a new connector. It's fairly simple if you're comfortable splicing wires. Here are the parts I used: https://www.amazon.com/dp/B0BCK9H2ZM https://www.amazon.com/dp/B09WQY83YN