OK, just had a look at my (2014 TS) wiring diagram.
You say that main beam works fine, so the relay and all the earths should be fine. Really the problem has to lie somewhere between the LH handlebar main/dip switch and the bulbs themselves.
I'd try the switch first. The fact that both bulbs are affected points to a common point. Is the bike outside in the rain a lot, or suffers from condensation in a humid garage? What's the mileage? Try working the switch back and forth to see if makes a difference, or see if you can get some WD40 inside.
If not that, then the wiring in between. There's several major plug/socket joints just underneath the tank at front RH side - one of those carries the HL wires. Have you disturbed them recently? If so, might be worth disturbing again and vaselining all the contacts (something I do anyway).
After all this, you have to trace the circuit from the switch to the bulbs with a meter, seeing that there's 12V getting to various points. If you've had the tank off recently, I'd check those plug/sockets carefully - they often seem to lie right on top of the cylinder head.