Adobe Audition changes my selection region before normalizing

I had to look at this several times before I realised what was happening. No, it's not a bug...

What you are applying is not an effect directly, but a Favorite. Chances are that the one you are applying has a time limit in its definition, and that if you select a time to paste that's longer than the duration of the Favourite, it will be truncated. The clue was that when you carried out the Paste, the duration was exactly what you'd previously selected. The jump in time occurs only when you select the Favorite. Chances are that if you look at the Favorite you are applying, one of the selection boxes (probably the second one) won't say Relative, but will have finite times in it and have an Absolute from Start value. For what you are doing, that wouldn't be correct. Can't be absolutely sure though of which one it is without seeing the actual parameters you are applying.