The entry will be removed, just be patient.
(If you want the shortest possible answer: incomplete == deleted)
Let's say "delete" is the wrong word for the action. What's really happening here is that the entry is manually set to the state "request sent, no answer" (thus "incomplete" ARP process) as if the machine would be really unreachable.
Now, the entry will be completely removed soon unless it gets a new valid ARP response in the meantime. In that case the entry would be re-added anyways even if it was removed instead of being marked as incomplete. So there's no actual pro or con to this behaviour.
But keep in mind that we're talking of a cache. Deleting things from caches is hard and expensive. It's way more efficient to invalidate an entry and wait if it gets replaced before it is finally removed. But for the system it's totally no difference if the entry is gone from the list or just marked incomplete.