Theres this weird corner that suddenly appeared, it is annoying

With Stage Manager enabled, you will see the new Windowed UI (User Interface) - and with this you will have different controls for the windows. The corner “handle” that you illustrate is used to resize the related App window; while Stage Manager is active, you cannot hide these elements of the UI.

With Stage Manager disabled (arguably, this being the far more useful mode when using the iPad without a connected external monitor), you will see the established Full, Split and Slide-over views listed and accessible from the three-dot button. In this mode, you will not see the handles of which you complain.

Stage Manager Multitasking views can be selected or disabled/enabled from iPad settings:

Settings > Multitasking Gestures > [Multitasking] - select Split View & Slide Over or OFF

You may find that the Split View & Slide Over mode is more to your liking - allowing use of multitasking features, but without the new windowed presentation of Stage Manager. The Stage Manager feature can also be simply enabled/disabled from the iPad Control Centre.

Move, resize, and organize windows with Stage Manager on iPad - Apple Support

That’s how you resize windows in Stage Manager

Move, resize, and organize windows with Stage Manager on iPad – Apple Support (UK)

Curious if the 'odd corner' item, goes away; when the small screen,

"Welcome to Apple Support..." is closed; this by using its small "X"?

If not, what else have you tried to resume normal appearance there?

Seems even a 'force-quit' ~ or restart, may clear items from display.

It’s perfectly normal already.

Move, resize, and organize windows with Stage Manager on iPad – Apple Support (UK)