Of course you can choose format for stem track in File > Render, but it is a long time. I made a rendering stem tracks on Shift + R. But default format (WAV 32 bit) gives too much load on hard disk, i think.
Project settings > Set media format for Apply FX, Glue, ... (at the bottom) also defines stem media format. Regards, e
heh, now render the correct, but I just rechecked checkboxes ... strange ...