I believe almost every bluetooth headphones on the market switches to a significantly lower audio when engaging two way audio (both mic and earphones). If your 2nd gen airpods really didn't do this while you were using the mic, that's kind of amazing.
The only thing I can think of is that there is software EQ compensation on Airpods to attempt to 'cover up' the bad quality of bluetooth in this two-way mode. It's one of the biggest weaknesses of bluetooth headsets imo. It's no joke, the audio sounds pretty bad during mic usage!