mining difficulty is not changed on regtest

I want to see the automated mining difficulty change on regtest but it did not change even I generated 2016 blocks. How can I see it?

// initial difficulty$ ./bitcoinA/src/bitcoin-cli -rpcuser=bitcoinrpc -rpcpassword=bitcoinrpcpass -rpcport=16591 -regtest getmininginfo{  "blocks": 0,  "currentblockweight": 0,  "currentblocktx": 0,  "difficulty": 4.656542373906925e-10,  "networkhashps": 0,  "pooledtx": 0,  "chain": "regtest",  "warnings": ""}// generate 2016 blocks$ ./bitcoinA/src/bitcoin-cli -rpcuser=bitcoinrpc -rpcpassword=bitcoinrpcpass -rpcport=16591 -regtest generate 2016// show the difficulty$ ./bitcoinA/src/bitcoin-cli -rpcuser=bitcoinrpc -rpcpassword=bitcoinrpcpass -rpcport=16591 -regtest getmininginfo{  "blocks": 2016,  "currentblockweight": 4000,  "currentblocktx": 0,  "difficulty": 4.656542373906925e-10,  "networkhashps": 12,  "pooledtx": 0,  "chain": "regtest",  "warnings": ""}// generate 2016 blocks again$ ./bitcoinA/src/bitcoin-cli -rpcuser=bitcoinrpc -rpcpassword=bitcoinrpcpass -rpcport=16591 -regtest generate 2016// the difficulty was not changed$ ./bitcoinA/src/bitcoin-cli -rpcuser=bitcoinrpc -rpcpassword=bitcoinrpcpass -rpcport=16591 -regtest getmininginfo{  "blocks": 4032,  "currentblockweight": 4000,  "currentblocktx": 0,  "difficulty": 4.656542373906925e-10,  "networkhashps": 12,  "pooledtx": 0,  "chain": "regtest",  "warnings": ""}

Article source: “”