Probably not an answer to your question, but some info about the databse tables.

The cf_users table includes all current users.  The pertinent values here are the sid and user_id values.

Then there is the cf_user_snapshot table, which also includes sid and user_id.  This will include multiple records for the history of the user as their username/displayname/email have changed historically.

Much of the history of instances, and I think currently assigned tasks - are through this cf_user_snapshot table.  That is why you can still see history for users who have been deleted and no longer show on the cf_users table.

When your user was on leave, I wonder if their record was removed from the cf_users table, and when they came back, it ended up making a different user entirely.  Perhaps search the cf_user_snapshot table for that user to see if the user_id value and sid value is consistent across the board.

Here's a query that might help: 

SELECT *
FROM [LFForms].[dbo].[cf_user_snapshot]
WHERE [username] LIKE '%matthew%'

Obviously, any edits to that table are high risk, and you shouldn't do it unless you know what you are doing.  But, if you find the user_id and/or sid have changed after they returned from leave, you could change those cf_user_snapshot records from the old values to the new ones, and that should carry over all their history and tasks assignments.

Unfortunately neither of those are an option for us at this time. What table(s) hold the data that shows for a users Completed Tasks?

That is probably what is going on. The user in question was on leave for 7 months and the 'Remove instance data from completed processes' is set to 183 days while the 'Remove instance statistics from completed processes' is set to 365 days. So she wouldn't have any completed tasks to show because they all would have been cleared out.