amp-ad set to sticky=top does not reappear after being auto-hidden

### Description

For an `amp-ad`, if we set the `sticky` attribute to "top", the intended behavior is that once the user scrolls to the top of the viewport, the ad disappears. This was fixed in #36378.

However, after the user scrolls to the top, if they then begin to scrolling down again, I would expect the ad to reappear, but it does not. Is this the intended behavior? It seems that would defeat the purpose of using a sticky ad and would impact revenue if the ad stays permanently hidden.

### Reproduction Steps

1. Create an ad that sticks to the top like so:
``````

2. As you start scrolling, see that the ad appears
3. Scroll back to the top and see that the ad disappears
4. Start scrolling down again and see that the ad stays hidden

### Relevant Logs

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### Browser(s) Affected

_No response_

### OS(s) Affected

_No response_

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_

Start by reproducing the four scrolling steps for an amp-ad with sticky="top" using the example in the issue, then inspect the amp-ad sticky behavior. Done means the ad disappears at the top and reappears when the user starts scrolling down again.

Written by the indexing model from the issue text.