How to stop upshift from 1st to 2nd while in 1st gear 2003 silverado

Hey guys is there a way to keep my truck from shift out of first while my gear selector is in the first gear location? I was goofing around today and had my tires covered in mud so I pulled out on the hard surface and pulled it to 1st on the selector and held the brake and hammered down, haha. Even though I was in first it still upshifted to 2nd gear while the gear selector was still in 1st gear. It won't hold low gear. I thought I had it set to hold first gear but I must of missed something.

Look at D0913 and D0928...

( M1 means Manual First lever position )

your D0913 is set to 255 mph, this should not allow the M1 1->2 upshift.
your D0928 is set to 39 mph, this will cause M1 2->1 downshift if you were in M1 2nd gear and are now slowing down to 39 mph or less...

D0928 having an attainable value may be allowing M1 2nd (somehow, I'm not sure how...)...

Try this: set D0928 to 254 mph.