One thing you should realize is that the specification NFR applies to the current arriving foot, however, the releasing of the toe doen't happen until you leave that same foot, which is one whole step later. Therefore, to determine whether or not to release your toe, you need to refer to the previous step.
For example, a common beginner mistake is to assume that the toe releases on step 4 of a Chasse from PP in Waltz (lady), because the arriving footwork is TH. But you need to look back to step 3, which was a toe, to realize that the left heel never lowers. So when the body moves away from the left foot on step 4, the right toe remains connected, and the left heel never touches down. In other words, no toe release on 4.
What the lowering on 4 of the Chasse does tell you is that the following step (step 5 of the Chasse, or 1 of the next figure, depending on how you see it) will have a toe release. This is true regardless of the footwork or rise & fall of the next step.
When you release your toe on the back step of a basic natural or reverse turn, it is not due to the NFR specification on that step. The reason you release your toe on that step is because you lowered on the previous step (toe-heel).
Let's say, for example, you have a back step that has foot rise, but the previous step was toe-heel. Would you release your toe? Yes. If you were to continue to draw the foot back, it might ultimately change to a toe to match the supporting foot, but initially upon leaving the foot, you would indeed release the toe.
I hope these descriptions make sense. When you understand the necessity to look back one step to determine the footwork of the foot you're leaving, you will always make the correct choice.
Regards,
Jonathan