There is a planned hard fork for Base Sepolia on April 20 at 18:00 UTC

https://docs.base.org/base-chain/node-operators/base-v1-upgrade

Base requires migrating from op-geth to reth for this hard fork. Firehose for reth is still under development/testing, and may not work. At the time of the hard fork new blocks may be processed incorrectly or not at all.

This is a testnet and as such it will be used for testing this upgrade path in preparation for mainnet update coming in May.