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@fathermaurer @BackwardsFeet @annyr

I got it.

https://github.com/WordPress/WordPress/blob/22f80e93829fe74c8ebace466f081d0b19e54b75/wp-includes/blocks/footnotes.php#L29-L39

We're checking if there are footnotes. If we think there are, we continue (great!). Then we decode them and count them. But, if the decode fails, there's nothing to count and... starting in newer versions of PHP... that's a fatal situation (vs just a warning, and it'd assume 0 count).

I'll open a ticket and push a patch this week to be more defensive, but would be good to know how un-decodeable content got there.