AMP Preview button is not showing up in WP 5.6, 5.7, and 5.8 initially

[delawski]

I've just noticed an issue: the preview button is not showing up in WP 5.6, 5.7, and 5.8 until the user makes a change and then saves a draft.

wp-5.6-amp-preview-only-appears-after-saving.mov

It seems the container is empty:

And also, when I load a post that is published I see the button show up for a moment and then disappear until I make a change and save the post:

momentary-dispay-of-preview-button-when-published.mov

It doesn't happen in either 5.9 or in 5.7 or 5.8 with the latest Gutenberg is active.

So I don't consider this a blocker for 2.2.1, but it should be fixed in 2.2.2.

Originally posted by @westonruter in #6881 (comment)

[dhaval-parekh]

✅ QA Pass

Test in WordPress 5.8 (Similar result found in WordPress versions 5.6 and 6.7)

Before

WP-5.8-Before.mov

After

WP-5.8-After.mov

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Similar results ( As shown in after video ) were found in the latest version the WordPress (5.9.2) with the new changes (and even without new changes)