Proxy settings are not passed to ant

I’m using Gradle 6.7 in Ubuntu 20.04 behind a corporate proxy requiring basic authentication to compile the JVARKIT.

Unfortunately, Gradle is not able to download required dependencies, regardless of the provided settings, neither via ~/.gradle/ nor via command line (-Dhttps.proxyHost=...) nor providing it via export ANT_OPTS=.
Then I enabled ant.lifecycleLogLevel = "VERBOSE" in build.gradle and I expected to see something like [ant:setproxy] Setting proxy to ... but it is not logged.

After adding ant.setproxy(proxyHost:['https.proxyHost'], proxyPort:['https.proxyPort'], proxyUser:['https.proxyUser'], proxyPassword:['https.proxyPassword']) to build.gradle I get the expected log line and Gradle downloads the dependencies.

IMHO, neither Ant nor Gradle uses the environment or command line proxy variables except ant.setproxy(...) is called. Is this a Gradle or Ant bug?