Sometimes, due to multiple reasons, subscription of a Web Push subscriber can become inactive. Some of the potential reasons are mentioned in this doc. When PushOwl sends a campaign to all subscribers, or a segment of your subscribers, we mark inactive subscribers as Inactive (Zombie). This leads to a dip in the number of active subscribers in PushOwl dashboard.
This is not a concern, since PushOwl automatically refreshes the subscription of those subscribers, when they visit the site again.