session files change to wav from mp3 without my intervention

You're both correct but MPEG 1 never really caught in in professional operations.  I know that when I was  converting our satellite transmissions to digital. We weren't willing to do that until MPEG 2 was available.

Either way, the basics of why not to use MP3 as a product format hold true.

You don't want to keep your files in MP3 if you're going to be editing them.  Audition (and every other DAW I know) works internally with wave files and there are good reasons for this.

MP3 is a very lossy compressed format and each time you convert to wave then back to MP3, the signal is re-encoded with more MP3 artefacts and distortions being added.  You can often get away with two of three of these stages unless you listen carefully and compare your work to the original--but you soon reach a point where the quality falls off a cliff and starts to sound horrible.

Why don't DAWs like Audition work natively with MP3 files?  Well, MP3 is a by-product of MPEG video coding--the full name is MPEG 2, Layer 3.  As such, the sound isn't stored as a continuous series of samples, rather, it's fed a frame at a time and reassembled by your decoder.  This makes accurate editing and most effects impossible to do with MP3 files.  You will find some MP3 editors online, but they limit you to level changes and very rough time edits.

MP3 is designed as a distribution format only, not for use in production.  Once Audition has converted to wave for you, you're best off staying that way through all your processes and only converting a finished edit/mix to MP3.  Even then, I keep a wave copy of my finished work before making an MP3 version.