This note-to-self is because it’s a really odd behaviour from Infopath so it needs not to happens again if I encounter the same so that I avoid banging my head for hours like I did.
I created a field called Field1 in Infopath 2010 with a value, and then a rule on a Field1 using the value of Field2.
The rule just didn’t want to show the Conditional Formatting as expected, even though I could see that all values were correct.
Suddenly I notice this message on top of the new Field1 :
Google that error message “Control bound to missing field or group” and I stumbled on this same error in InfoPath dev forum which just told me to not ignore it. (link to forum : Binding Error for Control – InfoPath Dev).
Enabling the field to load the default value just did the trick and now my rule works fine.
Damn Infopath ! :-)