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This is yet another example of WP’s faultless programming. This returns something that isn’t empty:
$footnotes = get_post_meta( $block->context['postId'], 'footnotes', true );
However, whatever that is, it isn’t valid JSON. WP assumes it is, thus the error.
After the line above, add this:
print_r($footnotes);exit;
What does it output?
You can stop the problem by copying lines 31-33, and pasting those below line 35.
I created this: https://github.com/WordPress/WordPress/pull/666
Then I found this: