That's disappointing. But what makes it especially frustrating is that the behavior is unexpected and thus reads like a bug. In my case, I spent an hour troubleshooting synchronization between Lightroom CC and Lightroom Classic before looking online, and discovering that it was "by design".
Given that, at minimum, it'd be nice to see a warning given when users attempt to add or modify keywords in Lightroom CC for photos that are synchronized with Lightroom Classic. That way, they don't make the mistake of updating their keywords in Lightroom CC in anticipation that they will be available later in Lightroom Classic. This could even be a one-time warning.
The goal is to ensure that the user is notified of the limitation since it's not otherwise intuitive or expected.
Thanks for taking the time to provide this feedback. I don't disagree that it's confusing to have Lightroom and Lightroom Classic sync with each other sort of, but not completely. Having said that, using both Lightroom and Lightroom Classic together in an ongoing fashion is not a workflow that Adobe recommends. And while the existing sync features in Lightroom Classic will continue to work, sync is not a focus for Lightroom Classic so it's unlikely additional sync features will be a...