110% of 2020 tax should prevent any penalty if you paid your estimates timely. are you sure this is an underpayment of tax liability and not something else because there would be no interest computed unless you noted that the balance would not be paid until after 4/18/2022
look at the 2210 and underpayment penalty worksheet to see if you can spot the reason for the penalty.
also, check the carryover worksheet to make sure the correct 2020 tax liability is reported
finally maybe there's a bug in the app
what you see as the interest rate on the underpayment worksheet is really the penalty rate
