I was having the same issue and was able to find the answer with the following StackOverflow article, excerpted here:
The PHP superglobal $_POST is only is supposed to wrap data that is either
application/x-www-form-urlencoded (standard content type for simple form-posts) or
multipart/form-data-encoded (mostly used for file uploads)
You need to fetch it yourself in RAW format with:
file_get_contents('php://input')
https://stackoverflow.com/questions/8893574/php-php-input-vs-post
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