Here are the reasons you would see a timestamp change for the "Last Interaction" in the client list:
When the client books something (a class or an appointment), including bookings made by staff on their behalf
When the client is added to a studio’s client list (e.g., staff manually creates/adds a client, lead is added)
When a guest booking creates a new guest client (guest reservations / guest checkout flows)
When a client is created through a third‑party booking integration
ClassPass bookings (when it creates a new client record)
Gympass/Wellhub bookings (when it creates a new client record)
When a client signs a waiver (waiver submission flow)
System backfill (not a real interaction, but it can update the field):
If a client record is missing “Last Interaction,” the system may fill it in using the timestamp of their most recent booking