Not a bug. It's a useful practice feature.
The practice feature (graduation hat icon) can automatically transpose the interval you set each time playback of the song or loop cycles.
There's also a setting that can increase the tempo each cycle by the amount you specify.
Sounds like you have accidently activated that. (set @12 BPM)
Make sure they are both set to OFF in the practice feature (graduation hat icon) whenever it's not desired for your practice sessions.
)BOB
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