When using the side panel editor, if the onPreSave hook returns an error a snackbar message if shown with the error and the field has a red error outline. However if this happens when performing an inline edit the snackbar message is shown but the cell reverts to a blue outline instead of a red outline like it does for other errors such as regex validation errors.
Without a red outline (or some other indicator) it's not clear to the user that the value in the cell has not been saved because there was an error.