I think there may be an issue with validation of a dropdown control when it's embedded on a subform.
I tested it by adding a dropdown control and a textbox control in a subform.
Then I created a main form, and added the subform. Then I also added another dropdown and textbox controls. All 4 controls are required. It sees to work fine for all controls on the mainform, but on the subform, the dropdown control doesn't work.
I have also attached the PMX file.