Look again at your piecing stitch picture on your machine. If it has 2 dots above it, it means you will sew in reverse when you hit the reverse / reinforcement button.
If it has just 1 dot above , it means you will only stitch a lock stitch when you hit the reverse/reinforcement button.
I don't see any straight stitch pictures with no dots above them.
Note that the reverse/reinforcement button is different than the start/stop button. Are you maybe forgetting to choose this when beginning to stitch?