The IMB's article 42 (pg 38) states (emphasis mine):
42 Power to disapply or modify export declarations and other exit procedures
(1) A Minister of the Crown may by regulations make provision about the application of exit procedures to goods, or a description of goods, when moving from Northern Ireland to Great Britain.
This is apparently in disagreement with what's been agreed to in the withdrawal agreement, where companies are expected to fill out export forms for all goods moving from NI to the mainland.
So that is in and of itself a problem, sure. It was agreed companies would need to fill out forms and now the UK is saying they may waive that requirement, in contravention of the withdrawal agreement.
But, is it a problem, really?
As far as I can see, this would merely be a hampering of the UK's economy, by theoretically allowing goods which should be tariffed on entering Great Britain to avoid those expenses. But isn't that a problem for the UK and only the UK, "shooting itself in the foot", as it were?
Obviously, if the problem were reversed, with the UK waiving a requirement that companies fill out export forms when moving goods from Great Britain into Northern Ireland, that'd become an obvious problem. After all, part of the rationale for Brexit was the hope of reducing tariffs. So if this were the case, companies would be able to import into the UK (paying low tariffs), move the goods into NI (without export forms), and from there disperse their products throughout the EU without paying its higher tariffs.
But as it is, why should any other nation care if the UK surrenders its right to tariff goods coming from Northern Ireland?