Consider the following Scenario:
Users created the Custom Lead and Custom Prospect forms and mark both as Preferred. However, when a Lead record is converted into a Prospect (manually or by creating Opportunity or Estimate), the form assigned for the Prospect record is still the Lead form.
Steps to repro:
1. Open Lead record in Edit mode.
2. Change Status manually from Lead to Prospect.
3. Click Save.
4. Click Edit again.
Lead is converted in Prospect record but Entry Form stays at Custom Lead Form.
Solution: Edit the Custom Lead Form and unmark the option Store Form with Record.