This is somewhat hard to reproduce.

Sometimes the output of a bat step is truncated. Given the following pipeline script :

node("Windows") {
    def output = bat returnStdout: true, script: "some_program"
    echo output
}

some_program outputs a file path, say C:\Jenkins\workspace\Project\file.zip

Instead, this prints something like C:\Jenk or C:\Jenkins\workspace\Project\fil for instance.

I use the following work around

node("Windows") {
    bat script: "some_program > stdout"
    def output = readFile("stdout")
    echo output
}

Which always work.