Does the downloaded transaction still have its status as New?
If it somehow got changed from New to cleared, then it will not have anything to match (note marking a New status transaction as "Reviewed" will clear the New status).
If that is the case (New status cleared) then you will basically have to do the merge yourself, in this case that might just amount to deleting the downloaded transaction.
When doing a manual match while using automatic transaction entry mode like it seems you are doing (clicking on the "Uncleared") the amount of the transaction you are clicking on and the downloaded transaction have to be the same. If they are different change the amount on the uncleared transaction to match the amount of the downloaded transactions and then select the Uncleared → Match to a downloaded transaction again.
Very strange, but I'm glad you found a solution.