I just noticed that some fabric just cuts strangely on the bias. I cut large (12") square, had the fabric on the 45 degree line, the ruler on the 45 degree line, the fabric taped on both sides, new rotary blade, and STILL got a wave in the middle of the triangle. Before even pressing it. (I checked on a straight line on the cutting mat) New mat, old mat, no difference. Even straight cuts go weird.
If you don't use steam, or press hard, how do you remove wrinkles? I find seams and whatever simply don't lay flat unless I do iron the heck out of them. All fabrics, too. What everyone calls good fabric to cheap fabric. Washed or not.
Seems to be stretched as soon as I cut it.
I've used all kinds of rulers. I have one from OZ that has a piece you line up with the dimension, a Truecut with the lip,
and the fabric still seems stretched or uneven on straight cuts.
Some piecing will look like the OP's, but I do have a 1/4" foot, and I've sewn clothing, and other articles without this problem. I've pinned, not pinned, you name it.
I can cut mat board for photos using a corked back ruler and exacto - that is straight. I've checked ruler edges and table - all flat and straight. All rulers have grips - either dots or a sheet.
The cutting table is high - but it's either high or my back. We also live in a very dry climate, and in the winter, static electricity is the pits.