urlwatch 2.28: html: separate: true not sending separate emails

I am testing the 'separate' feature with 2.28:
https://github.com/thp/urlwatch/blob/17d02c44e909b719b2b050b1c67cdbcf7d5fd0d8/docs/source/configuration.rst#L108

However, I am still receiving a single email that contains 4 changes:
4 changes: Camel Camel Camel, CNN1, CNN2, CNN3

Using the following setting:
```
  html:
    diff: unified
    separate: true
```


