This is due to the way card payments are processed (e.g. via Bancontact). Transactions performed via POM are not processed as regular bank transfers (SCT), but as card payments.
You can compare it with an online purchase in a webshop or a card payment in a store.
If you pay by card, the IBAN of the beneficiary and the remittance information will not be shown on your bank statements. Only some banks show this information, most don't.
However, behind the scenes POM makes sure the remittance info is included and sent to the beneficiary, so they can automatically link your payment to the correct invoice.