Bitcoin Core – Mempool not accepting transaction: ‘Reason for rejection’: ‘Insufficient fee’

I’m using Bitcoin Core and for some reason I don’t want to accept a transaction when there is already another transaction in the mempool, even if that transaction is using a different UTxO and has different non-witness data. Enough to cover transaction costs and fees. It also checks if the fee is greater than vsize. This means the problem is not the fees.

Can someone provide some insight into the reason for this specific rejection?