I just picked up my 24 LTZ 4 days ago. Got the Left Turn Signal Lamp message and the fast flashing this morning.
My previous 2018 1500 gave me similar messages and fast blinking after I had replaced one of the taillights. I doubled checked the connector and never saw it again. So my first attempt later today will be to do something similar on this new truck. Unplug some cables, double check the pins and make sure it's plugged in all the way. Along with leaving the battery unplugged for a good bit.
By the sounds of this and some of the other issues on the forum I'm reading about. I wouldn't be surprised that assembly lines are being rushed more than usual. Which is leading to half-assed work = Loose connections, bolts, half-filled liquids etc.
My local dealership appointments are booked through December. Driving 7 hours to the next closes dealership a day to get told "Can't duplicate complaint" is a hard pass for now.