By trial and error, I think I solved the problem. I had to press play first for some reason and then drag the files. If you load playbeat without pressing play it will not export the stems on the track. Can you try this on your end without letting playbeat play a beat and try to export a a stem.
That is correct, Playbeat has to render the sounds first (at least for a second) so the export engine is activated.
Glad you solved this and apologies for not mentioning this earlier, we will make sure this details is pointed out on our manual/tutorials.
Enjoy!
Max