Barclays has enforced an 18-character limit on the Beneficiary Name and Reference fields. If either of those fields exceeds that limit, then the payment will return status: Rejected or Cancelled.
Additionally, if the Reference field contains more than 14 numerical characters, the payment will be rejected.
Some accounts based in Jersey, Guernsey and Isle of Man are not eligible for Open Banking at the moment, and payments requests using those accounts will be rejected as well.