Student Loan Interest Question – Numbers Included

I recently made a large chunk payment of ~$18k toward my federal student loans and I’m confused about how the interest is being applied.

Loan balances and rates (current)

Total current balance: $20,064.24

Breakdown:

$5,337.85 — Direct Loan Subsidized — 4.740%

$7,460.62 — Direct Loan Unsubsidized — 4.740%

$7,265.77 — Direct Loan Unsubsidized — 5.250%

(Due date for active loans: 04/04/2026)

Payment history

03/04/2026

Payment: $434.52

Interest: $395.50

Principal: $39.02

02/14/2026

Payment: $18,044.82

Interest: $271.34

Principal: $17,773.48

02/04/2026

Payment: $431.86

Interest: $325.28

Principal: $106.58

01/04/2026

Payment: $431.86

Interest: $325.78

Principal: $106.08

12/04/2025

Payment: $431.86

Interest: $323.83

Principal: $108.03

11/04/2025

Payment: $431.86

Interest: $400.83

Principal: $31.03

The confusing part

Before I made the $18,044 payment on 02/14, my monthly payments looked consistent:

About $325 interest

About $106 principal

After paying down about $18k in principal, I expected my interest to drop significantly.

However, my 03/04 payment shows $395.50 in interest, which is higher than before the large payment, even though my total balance is now only about $20k.

Question

Why would the 03/04 interest ($395.50) be higher than the ~$325 interest payments before the large principal payment, even though the loan balance dropped by about $18,000?

Is this related to:

how servicers apply payments across loan groups,

billing cycle timing,

interest that accrued before the large payment,

or something else?