You have the dreaded FlexGate issue! Ouch!
Here’s more on the issue [https://www.ifixit.com/News/12903/flexgate|The Design Flaw Behind MacBook Pro’s “Stage Light” Effect] and [https://www.ifixit.com/News/13979/apples-2018-macbook-pros-attempt-to-solve-flexgate-without-admitting-it-exists|2018 MacBook Pros Try to Solve Flexgate Without Admitting It Exists]
Sadly Apple only offered a extended warranty for the 13” models [https://support.apple.com/13-inch-macbook-pro-display-backlight-service|13-inch MacBook Pro Display Backlight Service Program]
You’ll need to visit a real Apple Store and request a warranty exception. Be polite! And if they don’t give it to you, ask to speak to the service manager, and work your way up the food chain.
If you can’t get to an Apple Store your only option is replacing the display your self which is not that hard! Here’s the needed part: [product|IF117-064] and here’s the guide [guide|122604]