2018 F150 XLT. All signal lamps work, but when using left turn signal dash light blinks much faster and sound is different. Right side and flashers seem to be just fine. I've read these trucks no longer have flasher relays and it's all controlled by the BCM. Could this be a turn signal switch issue?

 

When you say all signal lamps work, are you sure the signal lamp flashing is the correct element in the bulb?

If there is a poor ground in the fast flashing direction , it could be using the other element in the bulb..

 

I'll switch out the bulbs to make sure thats not the issue.

 

The bulb may not the problem, usually it is a poor ground available to that circuit.which could be either the front or the rear.