I'm migrating from 7.5.4.
Unfortunately, the repository is private and not accessible.
However, I did a sample plugin using the legacy interface to reproduce the issues with uiState.
This plugin is accessible here : https://github.com/pchakour/kibanaUiStateIssues
To sum up, I had three issues with uiState. The first one was already here in 7.5.4, you can see my post about this issue : Empty uiState in a custom request handler. I found a workaround by getting the uiState directly in the rootScope.
The second, is that the request handler is doesn't call anymore after an uiState modification. And the last one is about the differences of behavior between editor mode and dashboard mode.
My sample plugin try to show these three issues...