beet update setting bpm to 0

I am not sure if this is a bug, but `beet update` is setting several `bpm` tags to 0. See logs:
```bash
arsaboo@arsmusic:~$ beet update
JAANE JAA (2026) - JAANE JAA
bpm: 140 -> 0
SADAK 2.0 (2026) - SADAK 2.0
bpm: 130 -> 0
The Definitive Collection, Vol. 4 (2022) - Aaye Kuch Abr Kuch Sharab Aaye
bpm: 76 -> 75
...
Fanoos (2021) - Takht Hazaar (feat. Riaz Qadri)
bpm: 90 -> 0
length: 5:10 -> 5:10
...

I am trying to understand the logic behind this and whether this is the expected behavior.

Start by reproducing the reported `beet update` behavior with the affected tracks and inspect the update path that reads and writes BPM tags. Compare the resulting values with the source metadata and existing expectations. Done means establishing whether changing BPM to 0 is intended and, if not, identifying the correction and a regression test.

Written by the indexing model from the issue text.