No, the trans is supposed to do that. If you are at a dead stop, and then select 2nd gear, then accelerate, the trans will be in 2nd gear unless you apply the gas to over 87.5% TPS.
Its called 2nd gear start, its to help 2wd cars and trucks get moving in teh ice and snow as it reduces engine torque to the rear tires.
Any 4l60e equipped vehicle will do this.
If you select 2nd gear at a stop, then go to WOT, the trans will downshift to 1st gear, then upshift to 2nd at the WOT upshift point, then if you continue at WOT, the engine revs will increase ntil the rev limit kicks in at 5600rpm for the Vortec 305/350 trucks.