When customizing row layout (in detail view or a modal), there are “quick styles” which contain advanced field groups. Those field groups can modify background color, font color, header formatting, capitalization, group border and few other things. (By the way, any ETA on when we will be able to create those ourselves? They look really promising.)
Apparently, those special formats also partially affect the formula editor.
And while you can’t directly edit field formula from this view, you can edit formula for the default value.
It is especially problematic when the font color is set to white, as it makes the formula editor completely useless: