Interest Assessed on Amended Return For ERC Credit Received Through Partnership

The interest due would be correct. The tax is technically paid late, as it applies to a previous year. In such a situation, interest is charged to compensate for the time value of money. However, you should have received interest income on the employee retention credit since it applied to a previous period also. Furthermore, that interest income was probably more than the  interest expense you are being charged for, as the credit was likely more than the tax on the additional income that resulted from amending your previous year income tax returns. So, you have to pay the interest but you probably received more than that in interest income.