What Arjan said. If your method is to make 16 bit wav files and then save as MP3 that’s fine, but you don’t need to. You could just as easily make 24 bit wav files and save as MP3. Or render to MP3 from the DAW 32 bit float with no dither if you’re comfortable with that.
“Why change from MP3 to PCM?” afaik, that’s what decoders do, even just to hear the MP3 file, whether it’s telling you it’s doing it or not.