reverse fading with adjustable fade?

Now that adjustable fade is available, can it be used to reverse a fade out, so that the volume throughout the fade is the same as if it were never faded?

I’ve tried selecting the whole faded selection and used adjustable fade (keeping all options on default except boost mid-phase to 100%). But there is a slight fade in at the beginning and fade out at the end.

You can’t completely and precisely reverse a fade out (or a fade in).
The Adjustable Fade effect will allow you to approximately reverse most of the fade, but not exactly and not all of it.

If you consider the very end of a fade out, it is silent - so it is impossible to amplify that silence back to the original audio. When the fade out is very close to silence, then a huge amount of amplification will be required to bring that back up to the original level. To reverse a linear fade out, you would need to apply an exponential fade in that goes from unity gain (times one) to infinite gain (times infinity), but that is impossible.