My tenant just received the UX update. At no point did the description of the "middle of the screen" form ever indicate that the form would now be much smaller and not able to be increased in size when using the sectioning configuration in the form editor. The "side panel" slide out allowed for 1 column or 4 columns at 100% zoom with comments closed. The amount of columns would increase or decrease based on the zoom and comments panel. The form was also responsive to devices and would be 1 column wide in mobile view. Additionally, the full screen view of a sectioned form readily allowed a "screen shot" for submitting form data to an external party. The current "middle of the screen form" would be fine - it shouldn't matter if it opens left, right or middle, but the ability to expand a window exists on almost any web application in use these days. It would seem that allowing for the ability then removing it so the form can be opened in the middle is a step backwards.
According to Microsoft, the reason for doing this was "We’ve heard from many of you that creating and editing items inside a side panel doesn’t make much sense; why relegate a critical, focused task to just one side of the screen? We’ve listened to this feedback and have elevated the core task of editing data in a list form to the prominence it deserves by placing it smack dab in the middle of the screen." How does MS come to this decision without pinging all users? Perhaps the majority would have rather had the screen open on the side if they could expand the screen than those who want it to open in the middle. I hope there is a way that a full screen mode is able to be reapplied in the future.