Using Setup > Customize Form we have a option putting eval conditions in Depends on section. Now, if I choice that the Field is Mandatory it will make the field mandatory even when eval is not true.
So, my suggestion is (or is there a way to do it that I miss - I know the new one frappe.toggle_reqd-)
if we choice Mandatory and some Depend on condition, the field should one be Mandatory then the eval is true.
For now, you should write custom code to make the field mandatory using frappe.toggle_reqd and there you should use the same condition which you have used in Depends On condition.