Tiagra brake lever won't return to neutral

https://reddit.com/link/1r6kyn0/video/b376jjyw0xjg1/player

Just bought this bike from Marketplace. It's a 2025 Specialized Allez, doesn't appear to have a lot of use/wear.

When I squeeze the rear brake lever, it works as it should. But when I release the brake, it doesn't return back to its normal position and the brake pads remain slightly in contact with the disc. When I physically move the break back up, the disc spins freely again.

I sprayed some wd-40 around the pivot points of the lever but that didn't seem to make a difference. How can I tell if it's a problem with the lever or with the hoses/cables? What other things should I try at home before taking it to a bike shop?