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For anyone to know the answer, they would have to have this same setup as you or be intimately familiar with Zapier's Slack connector. To venture a guess, it may have to do with the HTML entity characters. Maybe you have to pass them as entities and not characters.
Did you ask anyone one in a Zapier or Slack API related forum. Formidable only sends the data. If it's being rejected, you have to investigate what Zapier and/or Slack expect as data values. You need to check their API documents.