artifactory server object seems to ignore the bypassProxy setting by downloading in a pipeline

Download artifacts with an artifactory server object in a Jenkins Pipeline doesn't work in the Version 2.12.0 anymore.

Here is the error message:
...
[Pipeline] artifactoryDownload
Error occurred for request CONNECT <artifactory Server dns alias>:443 HTTP/1.1. Received status code 503 and message: Service Unavailable.
Attempting retry #1
Received status code 503 and message: Service Unavailable.
...

Both the Jenkins Server and the artifactory Server are in an Intranet that is why the Proxy Server is deactivated here.
We download artifacts in a Pipeline Jenkins Job with an artifactory Server object created in a groovy script:
...
def artifactoryRootURL = "https://<artifactory server>:443/artifactory/"
withCredentials([[$class: 'UsernamePasswordMultiBinding', credentialsId: <user id>, passwordVariable: 'artifactoryUserPassword', usernameVariable: 'artifactoryUserName']])

{ artifactoryServer = Artifactory.newServer url: artifactoryRootURL, username: artifactoryUserName, password: artifactoryUserPassword }

artifactoryServer.setBypassProxy(true)
def downloadSpec = """{
"files": [

{ "pattern": "<repository>/<artifact id>/<version>/*.zip", "target": "<workspace folder>/<download folder>/" }

]
}"""
def buildInfo = artifactoryServer.download(downloadSpec)
...

After downgrading to the plugin version 2.11. it works again.

Some additional Information:

• same error message after Setting bypassProxy to false (setBypassProxy(false))
• no problems by uploading of artifacts with similar code (setBypassProxy(true) too)
• similar error message by uploading of artifacts after setting bypassProxy to false
• the flag "Bypass HTTP Proxy" in the global artifactory plugin configuration seems to work: "Test Connection" shows "Found Artifactory 5.3.0" if it is set and "Error occurred while requesting version information: Service Unavailable" if not