I have had this happen a few times but never noticed why until yesterday. If you export a rule and the required expression is false then you have to enter the name of the rule to export it. Normally you just click the box twice, first it says export and the rule name and then it changes to Download and the rule name.json.
If the required expression is false, after you click export it changes to a blank and you have to manually type in the file name.
It's a minor thing, but I don't see why it would be that way.