Fix expiration_in() when called before first scan

[busyboredom]

Description

Fix expiriration_in() when called before first scan by using max of creation height and current height.

Checklist

• [x] Link relevant issue(s).
• [x] Manually test the change.
• [x] Ensure sufficient automated test coverage.
• [x] Update the README.md if necessary.
• [x] Update the CHANGELOG.md if necessary.