I just tried Cloudflare Pages for one of my websites and added a custom domain for the Page, which is already hosted on Cloudflare. While adding the domain, I got an information that there is an entry for that domain existing and will be replaced with a new one.
After confirming this, not expecting anything wrong to happen, all other DNS entries for the domain were gone! Important MX and TXT entries for email were deleted without any warning.
From the screen above I expected that Cloduflare would only replace the entry shown, and not all other entries for the domain.
If this is a bug, it should be fixed as fast as possible, because data loss is not cool, not even for a beta product.
If this is correct behaviour, add a big red box with a warning above the “Activate Domain” button which informs the user that all other entries will be deleted.