Really, upgrading from Mautic 4.4.13 to 5 is not a simple task.
I simply couldn’t upgrade one of my Mautic instances because it was an environment that had been updated since version 2.
In another instance, I encountered a similar error to yours, but in the campaign_lead_event_log table. In my case, the solution was to delete some records in the metadata column.
I recommend performing all these upgrade tests in a cloned environment and not in the production environment, to avoid your campaigns and everything else stopping from working.
This link - Getting json_array error after upgrade to v5 - #9 by viktor - has more information that may help you.
Best regards!
Sandro