I opened a support case at Microsoft an received this feedback
IoT Central currently discards invalid JSON payloads sent by devices. We are working on a feature to instead validate the JSON and mention the error on the portal so it is visible to customers.
A change was deployed on 2022-11-15 which contained an early push of that feature that was more strict in validation.
The original push for json validation has been rolled back so the issue will not be present for your IoT Central applications.
We have already taken the feedback and incorporated a change to our feature and merged that change to our codebase.
We have added support now to handle such cases (JSON array at the top level), and pushed the change to our codebase. Will be deployed in next release of this feature.