We're getting “Message not sent” every time we try to send a message so the user has to tap twice the send button for it to actually get sent.
Not connection related as it's stable. The issue is reproducible across multiple products, different geographic locations.
Zendesk version 1.2.0

Thanks for providing these details. Since the "Message not sent" error happens consistently despite stable connections and across different products and locations, it seems likely the issue is related to the Zendesk integration version 1.2.0.
If possible, try updating to the latest version of the Zendesk app or integration, as updates often include bug fixes. If you’re already using the newest version, consider reaching out to Zendesk support with your reproducible steps and screenshots so they can further investigate.
Also, double-check app permissions and message-sending settings within your platform to rule out any configuration issues.
If none of these steps resolve the problem, please feel free to contact our support team directly via the messaging widget—we're here to help!