If there is an overwritten parameter in the host or hostgroup edit page and an other parameter is overwritten (Override button) then randomly an existing parameter gets changed.

Steps to reproduce:
1. Create parameter for hostgroup: Name: test, Type: string, Value: test
2. Create parameter for host in hostgroup from above: Name: test2, Type: string, Value: test2
3. Override parameter test for host: Now the parameter test2 has been removed and an empty parameter has been added.

Let me know if you need further information.