iPad Models Reverse Charging

Today, I was charging my iPad (air, 5th gen), and I plugged it into my power bank (i did use a apple cable if that makes a difference, usb c to c as well ), and instead of my power bank charging my iPad, my iPad charged my power bank. Apparently it is reverse charging or something. I talked to Belkin support (manufactures of power bank) and Apple support. Belkin offered to replace it, and Apple said take it into their stores. We tried a lot of troubleshooting steps, and nothing fixed it. It wasn’t a percentage thing (my ipad was 8 percent and the power bank was 100). I also did some research, and apple doesn’t not have any feature or button to change it. I tried with my other power bank (not sure what brand, just got it off amazon), and it worked. The other power bank is 32k mah, and the belkin one is 20k mah. Although the belkin one is smaller size, it is also incredibly powerful (+ the amazon one has prob worn down a bit). Anybody had this happen to them?

edit: just tried with the belkin power bank a few more cables. usb c to apple watch works, usb c to lightning works, and my ipad does charge with the usb a to c.