Hi partner,
Are you mean this problem?
If so, it is by design.
The UI of D365 is now the unified interface.
Unified Interface uses responsive web design principles to provide an optimal viewing and interaction experience for any screen size, device, or orientation. It brings all the rich experiences to any client that you are using. Whether you are using a browser, tablet, or phone, you will be able to consume similar experiences. (Form this document)
In other words, the unified interface will automatically adjust the form layout to allow different sizes of devices to get the same reading experience.
The preview is on the classic UI, so its form layout strictly follows your design. But it is not the unified interface, so the style in the unified interface is different from the preview.
In the unified interface, if you lower the zoom of the browser, then you will see the four columns you designed. Or if you change the computer screen to a higher resolution, you can see four columns side by side.
I would recommend you edit the form in Power apps, not in the classic form editor.
Go to https://make.powerapps.com/, select D365 environment, click Data > Entities, select the entity.
Click the form, and you will enter the new form editor, this preview effect is unified interface and real-time, what you see is what you get.
