How to set the PATH environment variable in Jenkins configuration on Windows?
It needs to be "Path", not "PATH".
Jenkins treats this special variable in a case-sensitive way, and only "Path" is recognized as being the path variable. "PATH" looks to jenkins like a generic environment variable, even on Windows.
The issue I had was caused not by a wrong Path configuration. %PATH%;D:\path\to\COMPOSER_HOME\vendor\bin is correct.
don't get to confused about %PATH% vs $PATH in the Value field. While %PATH% is the correct syntax for windows nodes, you should use /foo/bar:$PATH to extend PATH on unix nodes