Laptop have black shadow bands arising from bottom of display

This is only noticed on dim screen as the brightness is increased this will not be noticed anymore. Is this any issue with my laptop display because it is in warranty.

It's certainly not a normal behavior and might indicate a problem with the monitor's back-lighting.

As it's under warranty, I wouldn't think twice and demand a replacement.

Only if the same problem occurs with the replacement laptop, the conclusion would be that it's a general problem with this model. It's then up to you to demand whatever you can under the warranty, if possible up to asking for reimbursement.

(Just my personal attitude: In such a case I usually ask for reimbursement and buy another model. I don't like a model that already has a problem out-of-the-box, and asking for a replacement might make a reimbursement impossible.)