The new functionality to have the in-cell editing validate based on the model doesn't work with fluid validation. We use a custom component that integrates fluid validation with blazor which works fine for all controls including telerik input controls, but the built in validation for the grid editing doesn't seem to work with it.
Also there seems to be a bug with the validation using standard attribute validation. If the row you are working on has an existing record that does not validation properly, the in-cell editing will stop working once you go in to any cell and make a change. The edit mode on that cell will close as if everything updated ok, but after that, you will not be able to click into any cell to edit. While normally the data should already be valid, there can be scenarios where new validation rules are applied and existing records would become retroactively invalid.