We had this issue inside of a Global Library but are able to reproduce it with a simple pipeline job.  

Steps to Reproduce:

1. Create a pipeline job with the following script:

   //Not the real function but good enough to demonstrate problem
   def getSlaves() { [
       [name: 'good', isOnline: true ],
       [name: 'bad' , isOnline: false]
   ]}
   
   @NonCPS
   def verifyAllSlavesOnline() {
     def errorList = []
     for (def slave in getSlaves()) {
       if(!slave.isOnline) {
         errorList <<  "Slave is off line: ${slave.name}"
       }
     }
     errorList
   }
   
   println verifyAllSlavesOnline()

1. Expected output of println is

   [Slave is off line: bad]

   but instead with get:

   [{name=good, isOnline=true}, {name=bad, isOnline=false}]

   Essentially the return value from getSlaves() is being returned instead of the content of errorList.

In our case, the use of @NonCPS here was in error because the original code that required it had been refactored into the getSlaves() function. So we were able to solve the problem with this rewrite:

//Not the real function but good enough to demonstrate problem
def getSlaves() { [
    [name: 'good', isOnline: true ],
    [name: 'bad' , isOnline: false]
]}

def verifyAllSlavesOnline() {
  def errorList = []
  def slaves = getDrSlaves()
  for (def s = 0; s < slaves.size; s++ ) {
    def slave = slaves[s]
    if(!slave.isOnline) {
      errorList <<  "Slave is off line: ${slave.name}"
    }
  }
  errorList
}

println verifyAllSlavesOnline()

The above produces the expected output.

Decorating the corrected function with @NonCPS again causes it to return the same wrong results!