I'm not entirely sure what is happening but it appears that when these methods are called it only runs the closure once and doesn't return the expected values.

def test() {
    l = ['a', 'b', 'c']

    r = []
    for (String v : l) {
        r.add(v)
    }
    println r.join(' ')


    r = l.collect {
        it
    }
    println r.join(' ')
}

test()

If I run the above with command-line groovy it does what is expected:

$ groovy test.groovy
a b c
a b c

If I put this in the vars/ directory of a global pipeline library and call is as "test.test()" only the first loop works:

[Pipeline] echo
a b c
[Pipeline] echo
 [Pipeline] End of Pipeline
Finished: SUCCESS