Multiple Issues with Expression Builder (not saving, backspace selects all data)

I started with a simple CONCAT(arg, arg) in a select statement. I selected the CONCAT(arg, arg) and entered the expression builder. (My intent was to replace the CONCAT with a DECODESTRING.) While entering the data, I mistyped a string and, without looking, backspaced a couple of times. What happened is the first backspace selected the text field and the second deleted it all.

After retyping it (more carefully), I applied and OKed the expression. The transformation editor did not reflect my change. If I re-enter the Expression Builder, I can see my new DECODESTRING function. I just can't get it where it belongs--in the transformation.