How Does EIP-1884 Relate to the Gas Costs of External Calls?

EIP-1884, part of the Istanbul hard fork, increased the gas cost of certain opcodes, including SLOAD (storage load). This change increased the operational cost for contracts, potentially causing existing contracts that relied on the 2,300 gas stipend from transfer to fail.

This is a prime example of why relying on a fixed gas limit for security against reentrancy is fragile.