I would verify that the brake isn't too tight or that it isn't slow to release. This condition requires lots of extra torque hence the lunge. Be sure also that the rear idler isn't partially tight and that the VS pulley works freely. BTW, you are really spinning your wheels if that belt you've replaced is not an MTD belt.

It is the correct type of belt, fully jacketed OEM.
Where is the brake adjustment located?
Thanks.

might try looking in the cavity under the tilting seat - get the full model and serial numbers - go to MTD manuals website - see if you can download an owners manual - brake adjustment procedure should be found there...