Hi there SusplssBE,
Thanks for reaching out to Apple Support Communities. It sounds like your Apple Watch had a red line showing on the display so it was replaced. The replacement has the same red line. We'll be happy to help.
When you got your replacement Apple Watch did you set it up from a backup of your previous Apple Watch? Do you use a case or screen protector? If so, did you put that on your new Apple Watch?
You can try erasing your Apple Watch, then setting it up as a new device, leaving off the backup. Test to see if you have a red line on the display. If you don't, there may be an issue with that backup. If it still appears, there may be an issue that needs repair.
Make sure you have a recent backup of your iPhone. Then use this page to erase and set up as new: Unpair and erase your Apple Watch.
Here are the basic steps to set it up: Set up your Apple Watch.
Reach out here for further assistance: Contact Apple Support
Take Care!