OK, a colleague was able to test in 5.4 and the problem is still happening. Let me reiterate the steps once again in case I left something out. I should point out that we are working with Custom Lists, here, not document libraries, in case that makes a difference.
Also, my test used a yes/no field rather than a drop-down, so you probably want to try reproducing with a yes/no field just in case there is something special there (but I doubt it).
Using your example screenshot above (assuming you have a custom list and not a document library):
- add a yes/no column to your list; let's say it is called "Picker".
- ezEdit your Create form, and add Picker to the bottom of the webpart.
- Enable auto postback on Picker.
- Now add a new item to your list. Fill in the fields above Picker – perhaps skip over your drop-down field for now, and enter alpha characters in the Weight field.
- Now click the yes/no field, triggering the postback.
In our install of 5.4, on postback, all the fields disappear and the upper and lower button bars "collapse together" and look like the following:
