Why Can Fee Estimation Be Inaccurate during Periods of Sudden Network Congestion?

Fee estimation algorithms rely on historical data and may not react quickly enough to sudden, sharp increases in transaction demand. If a large number of high-fee transactions flood the mempool simultaneously, the estimated fee based on recent averages will be too low.

This causes the user's transaction to be underpriced and delayed until the congestion subsides or they use RBF.