There can be multiple reasons why you have deleted clients in your session bookings.
The simplest explanation is that you deleted the client from your client list. Usually this removes all of their future bookings as well but a glitch might have occurred.
Your client might have disconnected from you and reconnected to a deleted account in the past. If that happens, ask them to disconnect on their end and request a connection on the Member App.
If your client reactivates a legacy client account (set up prior 2022) and you deleted that client ever since, they might be able to use it and book into your sessions. If that occurs, try to identify the client and ask them to reconnect with you, either by disconnecting on their Member App and requesting to connect again or by using a more recent account they might have created.