Can a Miner Include a Lower-Fee Transaction before a Higher-Fee One?

Yes, a miner can technically include a lower-fee transaction before a higher-fee one, as the protocol does not enforce a strict fee-based ordering. However, it is economically irrational for a profit-maximizing miner to do so when block space is scarce.

In a congested network, a miner's software is typically configured to prioritize the highest fee-per-byte to maximize revenue. The only exceptions might be due to a bug, a custom or altruistic policy, or if the miner is optimizing for a factor other than immediate fee revenue, such as Maximal Extractable Value (MEV).