When opening the form designer, an inconsistent experience might occur if a form designer is already open on another browser tab. The form designer might display inside the platform instance window when form designer is open and the user configures a label.
Steps to Reproduce
Go to a record form.
Right-click in the header and choose Configure > Form design.
Do not close the Form designer page.
In the change form, configure the label on the Number field and click Save.
Note that the form designer page has loaded inside the platform instance
Closing the previous form designer page. The new designer page will show correctly.
Related Problem: PRB637362