Too loose, need diffferent yarn?

I’m an intermediate, continental knitter, and I’m working on a “wave” pattern sweater from tahki stacy. The pattern is worked in k2, p2 ribbing, with a M1 row worked into every other k2 rib, then dropped to create a ladder every 7 rows. The pattern calls for 100% cotton, but I’m using caron simply soft which is acrylic. When I drop my M1 stitches to make the ladders, they are very stretched out and won’t stay flat. Do I need to switch to a cotton yarn, or will the ladders shrink and flatten when I block and wash the sweater when it is finished?

I can’t say for sure, but I suspect that after dropping the stitches, even the cotton wouldn’t be flat. I think that once you wet it, it will straighten out.