tnkmt

It's not a bug, it's a feature. Here it is explained: https://github.com/spesmilo/electrum/issues/5797#issuecomment-557942439

It's better **not to use** OS installed on a hard drive. It's better to disconnect the hard drive physically when generating keys. In case there is a private key stealing...

You can use this script: https://github.com/tnkmt/brute.bip39