Yes — switching an automation out of Testing mode clears the recipients that were collected while it was in Testing. Testing mode is meant for verifying your automation's logic and setup, not for building a real mailing list, so those test recipients are removed once you move to Enabled (or to Inactive). This is expected behavior, not a bug.
If you need to keep working with real recipients, don't rely on what accumulated during Testing — once you enable the automation, it will start collecting recipients fresh under its live rules.
Also note: you can't switch an automation directly from Enabled back to Testing. If you need to return an automation to Testing mode, you'll need to pause it first, then switch it to Testing from there.