Mystery Sandbox Account Shows Up In Settings

You left several important parts out. Of course, if those parts contain personal information, you shouldn't include them. But by the same token, you can't expect any accurate response regarding items that no one has any idea about.

I can tell you that developers can setup sandbox accounts for testing apps. These accounts aren't real iTunes accounts. They can only be used for accessing purchased apps or content for an app while running in developer mode. Developers need to be able to test various parts of an app, including the purchase flow, and other activities that involve purchased iTunes content.

Apple Developer documentation explains it in more detail:

In iOS and iPadOS, the sandbox account appears in Settings > App Store after the first time you use the device to attempt a purchase in a development-signed app. There’s no need to sign out of the non-Sandbox Apple ID. Sign in using a Sandbox Apple ID.

It certainly sounds like you are involved in testing this mysterious app. Mixing sandbox and live AppleID accounts can scramble them. You don't want to do that.

tabletipper wrote:

Admittedly, I could of laid out the scenerio a bit more cohesively.

Whatever apps you’re testing with here while using Xcode have likely created these sandbox logins.

If you are not now and have never tested the mystery app, then you’ll want to check back with Apple developer support, and with the mystery app vendor.

We’re not Apple Developer Support, and we have no insight into your recent and past activities and testing, and can’t access your Apple ID and developer activities. Put differently, these are all things that no one here has no idea about. And if there’s possibly been any side-loading or iOS-security-relevant activities here, the realm of possibilities increases yet further.