@Erik Lundgren - Interesting observation. I always new the Text object would auto-expand horizontally, but I never paid too much attention to the conditions that lead them to expanding vertically.

A few things I just noted:

• All still-auto-expanding text boxes eventually max out in length and start expanding vertically.
• The two conditions that appear to force vertical expansion are either:
  • a change in size to the text object by dragging it horizontally or diagonally, or
  • pressing the Enter key.

All of that being said, 

Is there a way to … after having manually resized a text box … to make it go back to auto adjust horisontal size based on the amount and size of the text?

I cannot find any way to force it back to its original state. I tried:

• Duplicating it, to see if the new object would act like “new”.
• Using the Copy/Paste Style feature from a brand next Text object to the now vertically expanding.

Stumbled upon this topic after googling how I can set a text back to auto-fit. Really weird that it isn't possible and it sounds more that this is a missing feature or a forgotten implementation.