You mentioned 1x6’s, which may be the reason for the different experience. My supplier carries boards, usually for shelving, in 12" x 3/4" x 120" sizes. I believe they are kiln dried. All of the wood I receive is nearly always perfectly flat. I usually let it sit for a few months to over a year, just depends on how much I’m using, and where it was in the stack. When used for general construction it’s perfectly fine. All of the warped boards I’ve experienced were flat when they were cut. Usually after being cut and sealed I start to see warp about a week out. Since my shop is humidity/temperature controlled I didn’t see a lot of this until customers told me about it weeks later, so I started holding onto finished work longer to see it develop. I work in Texas where it is generally hot and dry, but that’s not always the case for where the signs end up. It could also be the local wood and the climate it was grown in as well. This evening I’ll try to find a few discards with the kind of warp I’m talking about so you can see what I’m talking about. The only thing I can imagine causing it is the cuts relieving internal tension and the sealing causing uneven moisture retention.