I see a mention of SENDENV in ssh configuration options. Have the ssh configuration options been modified in the account that runs the Jenkins agent?
If that hunch is correct, then you could switch from ssh authentication to https username/password authentication or you could use a Bitbucket access token with username.
There is also a mention of SendEnv in the git ssh source code to communicate the git protocol version. Since git protocol 2 is a relatively recent addition, that might be involved in the issue. If that were the issue, it would also be resolved by using https rather than ssh.
I'm using portable git for Windows 2.31.0 to access Bitbucket from 5+ different Windows machines with ssh and https authentication. Both are working for me.