Did you try adding the New button to the same exact grid? Usually, that flickering is related to a missing useMemo, not to the New button presence.
For the second issue, the toolbar is only added when the New button is present, as there’s no other content to show and an empty toolbar doesn’t render correctly. You can open a feature request in Hilla to allow to add further content, so that the toolbar is created if any content is to be put into it.