It appears that tab + Enter does not work in Kickoff in all areas. When tabbing through the interface I selected the session icons for sleep, restart, shut down and they would not activate when hitting ENTER. However, tab to select applications did work when hitting ENTER. Wayland, latest unstable build. Neon.
When using keyboard navigation, Enter isn't the button that triggers the highlighted item; it's the Spacebar. This is pretty much universally true everywhere (except for in Elisa, where we deliberately break this because the spacebar should always trigger play/pause regardless of focus). So I'm afraid at the moment, this is the expected behavior.
Can we say that it should always be the space bar and "not" the enter key? Because then, right now, the enter key works to launch items in kickoff. I think you're saying that it should be the spacebar.
The general rule is: when navigating with the keyboard, the spacebar will activate the highlighted item. This is baked into Qt's keyboard navigation system. Sometimes, the Enter/Return key will also do it, especially in item views where items are commonly navigated between using the karrow keys (e.g. KRunner). But never for UI controls like buttons.