Ah OK - I don't use forms via email. All my forms are used via SharePoint and web pages.
Your rules on the dropdown were formatting rules and incomplete.
A/Your drop down automatically clear on a change of value.
I add a dummy field to see what was happening to the drop down box when I selected a new value, it was changing it's value as expected.
If email acts differently. I've added a 'clear' rule to the attached form. The box you want to look at and see a "value" in would be the RequestTypeValue field not, your drop down control.