Migration from 2.7 to 3.5 and random URP

Hi,

I'm attempting a migration from 2.7 to 3.5.0 and the way it's recommended
in the documentation.

1. Freeze the following settings
inter.broker.protocol.version=2.7-IV2
log.message.format.version=2.7-IV2

2. Upgrade kafka version to 3.5.0
3. Change inter.broker.protocol.version to 3.5
4. Change log.message.format.version to 3.5

After step 2, I get random under-replicated partitions on some topics (all
of them are empty). But the way they are under-replicated seems weird to me:

```
Topic: __consumer_offsets Partition: 13 Leader: 10001 Replicas:
10001,10000,10002 Isr: 10002,10000
```

It turns out the leader is no longer part of the ISRs. I didn't even know
this was possible.
Does anyone know how this is possible and if this is a known issue?

Kind Regards,
J