Sharing my solution just in case it helps someone since it wasn’t related to rogue 3rd party plugins or bad code in the functions.php file.
In my case the issue was due to me not adding a field label to the flex-content field in order to avoid redundancy in the admin UI and by extension, the field name was mistakingly left blank. Started working after adding a field name (label was still intentionally left blank).
Super weird that field name isn’t marked as required when creating new fields.
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