This is not the dev's main job, and they don't have a patreon, so it's understandable that they develop at their own pace. And they don't really disappear between updates, in fact, they're pretty active on itchio, so I'm confident that neither games are abbandoned.
That being said, I remember reading that the next update is going to be for Prowler, not Grove, and it's estimated to be finished in the middle of this year.