The warping is real and it's mostly about bias. When your drafting puts narrow strips at odd angles, you're often cutting fabric on the bias or a partial bias, and those edges stretch just from being handled. Cutting to a sixteenth doesn't help much beacuse the fabric moves between the ruler and the moment you sew it.
Two things fixed this for me. First, starch your scraps before you cut anything. A light mist of spray starch or even a bit of sizing, let it dry, then press. Stabilized fabric basicly stops behaving like ribbon and holds its shape while you trim and handle it. I do this now for anything with a strip under about an inch wide.
Second, and this is the bigger one: stop pre-cutting precise pieces at all. In foundation piecing you only need fabric big enough to cover the section plus a seam allowance. Cut chunky rough pieces, pin or hold the fabric on the unprinted side of the paper, fold the paper back along the seam line, and trim the excess with an add-a-quarter style ruler against the folded edge. That way every piece gets trimmed to exact size right before it's sewn, and the paper itself is your template. All that measuring down to sixteenths on paper is only useful for the big background sections where you can afford to cut generously.
Also check your pressing technique. If you're sliding the iron around on those little pieces you're stretching them. Press straight down, lift, move, press again. And drop your stitch length a notch or two, since short stitches perforate the paper less and stop the seams from puckering on skinny strips.
One more thing on the 30x30 size: with that many tiny sections, do a test sew of just the smallest unit before committing fabric to the whole block. If a single unit comes out even a hair off, it compounds thru the whole layout, and it's much easier to fix your seam allowance on one frog eye than on a finished block.