I have an audio file of a song that I'd like to learn and have dragged into Studio One and at some point may want to mix BiaB content with it. I don't know the exact tempo of it but suspect it's somewhere around 83 BPM. S1 has a time stretching capability that I have used in the past and therefore believed that I understood it; but apparently not. Here is the procedure that I have successfully used in the past.

1. Drag in the audio
2. Display “Bars” in the Time Ruler
3. Drag the beginning (first down beat) of the song to Bar #2
4. Activate the Metronome
5. Play the audio and adjust the tempo to match
6. When happy with the tempo, right-click the audio and double-click value in the File Tempo field (it may say “Not Set”). Type in the tempo value. The Speedup field should be set at 1.0
7. To change the tempo of the audio adjust the Speedup value.

My problem starts at Step 5. I can get the tempo of the song and the tempo of the metronome to match only for the first few bars, then they drift apart.

Note in the screenshot that bar 2 of the waveform is aligned with bar 2 of the ruler and the same for bar 3. Bar 1 is reserved for the 4 beat count-in.
This is from the manual.
Automatic timestretching is based on the relationship between the Song tempo and the audio file’s tempo.
Each Audio Track has a Tempo mode that controls the behavior of the Events on the Track, based on the Song tempo. The Tempo mode can be selected in the Track Inspector. The following modes are available:
•Don’t Follow Events on this Track are independent of the Song tempo. They are never moved or stretched automatically.
•Follow The start positions of Events on this Track are tied to the musical grid. Thus, the Events move when the Song tempo changes but they are not stretched.
•Timestretch Event start positions follow the Song tempo, as in Follow mode. In addition, the Events are stretched to fit the Song tempo.

Any idea what I'm not understanding here?

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BiaB 2024 Windows
For me there’s no better place in the band than to have one leg in the harmony world and the other in the percussive. Thank you Paul Tutmarc and Leo Fender.