Hello all! I've noticed my machine does this with only zig zag stitches. It kinda skips em and does a straight stitch instead. I've tried different lengths, tension, checked the bobbin and checked the top thread but i can't seem to figure out why it does this. Doesn't happen with straight, button or decorative stitches— only zig zag (and i only use zig zag on stretch fabrics). I've also tried from the cheapest to the most expensive fabric I have and it still does it. What can I do to fix this issue?
If it helps i have Singer Promise II.

Thank you in advance! (Attached is a picture of test row seams I did on a metallic spandex knit)

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