Someone reported this on Stack Exchange, and it's pretty easy to replicate with their steps: "when you edit a pending pay later contribution, add a check number, change status to complete and then i go to search contributions, enter the payment method and search by check number i get no result."

check_number is a field both in civicrm_contribution and civicrm_financial_trxn. I suspect this is for historical reasons rather than anything anyone considers correct in 2019.

• To solve the immediate issue, "Find Contributions" should search the check number field of related financial transactions rather than the contribution itself.
• I think there's a strong argument that civicrm_contribution.check_number should be deprecated altogether, but that's a much larger issue.