Sorry to bump an old post, but I’m having this issue as well, on Drupal though – not WordPress. I had previously disabled Content Security Policy to try and fix this issue, but now I see the 422 Unprocessable Entity when clicking the Show Full Post button. This broken button functionality seems to be happening across all articles that have been posted, so not just new ones. Older ones with truncation enabled are also affected.
My install is behind Cloudflare, but I’m not seeing any security event logs that indicate resources are being blocked from the WAF area or managed rules. I’m really not sure what to do next to fix this.