The issue can be replicated in the SEED folder.
To ensure correct IBAN generation, the bank format setup must include the 4-character alphanumeric bank code prefix (e.g., MIDL) when IBAN management is enabled.
In the GESTCY function (Countries), the bank account correct format should be: 4A[-]2#[-]2#[-]2#[ ]8#
In the reported case, the customer was using: 2#[-]2#[-]2#[ ]8# — which omits the required 4A prefix.
This is a common oversight, as many customers do not include the 4-character bank code in their configuration. Updating the format to include the prefix resolves the issue.