Where did you turn timestretch off, playback setup or file attributes? I’m pretty sure the playback setup settings override whatever the file attributes settings are for timestretch.
I just tried it on some tracks and was able to get them to play at their original BPM no matter what the octatrack is set to. Pretty cool hearing the drums speed up while the bass track stays the same!
You also might try rendering out each song as its own section.
I am playing long drum, bass, and instrument rendered stem tracks without any problem here and I can also slice them into sections and timestretch to differentBPM’s if I want.