I understand the concept that a grade retains the visibility status of the policy in place when it was entered. However, if a grade is deleted, THEN the Grade Posting Policy is set to Manually, THEN the grade re-entered, the grade is visible. Here's 2:43 minute video showing the behavior.
This behavior doesn't look intentional and should probably be reported to Canvas support if you haven't already done so. They can investigate further for you.
Here's the response from the case [#05003746] I submitted. Edited for brevity.
We tested this in our own test course also, and we believe that this intended behavior. The reason that this is intended, is because of that same logic. Even though you've set the grade to - (Nothing) our system will notify that student to let them know that their assignment grade was changed, and will show inside of their gradebook.
You will also see that as soon as you've put the grade posting policy to Manual, you will have the option to hide grades before entering in any grades. Even with a grade of nothing, our system recognizes it as a grade that was entered even if you changed from points to nothing. This is the effect that you're seeing when you put it in the Manual grade posting policy.
I can definitely see how this could be frustrating though, and I would recommend making a feature request to have this functionality change.
I followed the suggestion and created an idea which you're welcome to vote up.