Steps to reproduce:

1. Add a step in the scenario
2. Fill required fields (name, url, etc...)
3. Use Post type as Form data
4. Add more than one Post fields
5. Add Required string
6. Save/Update step and scenario
7. Open scenario and the step and update Required string
8. Save/Update step and scenario
9. Open scenario and the step again

Result:
Post type changed to Raw data and the field will be empty.

Expected:
Keep Post type as Form data and the fields

I think it is related to below ticket, our customer is asking workaround and the exprected date for fixing this issue.
https://support.zabbix.com/browse/ZBX-15585