I'm appearing to hit an issue where a released resource does not get an update on the environment variable once the lock is released.  See the following code (with only two resources available in the automation-accounts label)

node {
  parallel (
    "p1": {
      lock(label: 'automation-accounts', variable: 'ACCOUNTS_VAR', quantity: 1) {
        echo "A $env.ACCOUNTS_VAR"
        sleep 4
             }
    },
    "p2": {
      lock(label: 'automation-accounts', variable: 'ACCOUNTS_VAR', quantity: 1) {
        echo "B $env.ACCOUNTS_VAR"
        sleep 2
             }
    },
    "p3": {
      lock(label: 'automation-accounts', variable: 'ACCOUNTS_VAR', quantity: 1) {
        echo "C $env.ACCOUNTS_VAR"
             }
         }
  )
}

In the log I get the following (note "C null"):
[Pipeline] {
[Pipeline] parallel
[Pipeline] [p1] { (Branch: p1)
[Pipeline] [p2] { (Branch: p2)
[Pipeline] [p3] { (Branch: p3)
[Pipeline] [p1] lock
[p1] Trying to acquire lock on [Label: automation-accounts, Quantity: 1]
[p1] Lock acquired on [Label: automation-accounts, Quantity: 1]
[Pipeline] [p1] {
[Pipeline] [p2] lock
[p2] Trying to acquire lock on [Label: automation-accounts, Quantity: 1]
[p2] Lock acquired on [Label: automation-accounts, Quantity: 1]
[Pipeline] [p2] {
[Pipeline] [p3] lock
[p3] Trying to acquire lock on [Label: automation-accounts, Quantity: 1]
[p3] Found 0 available resource(s). Waiting for correct amount: 1.
[p3] [Label: automation-accounts, Quantity: 1] is locked, waiting...
[Pipeline] [p1] echo
[p1] A <Account1>
[Pipeline] [p1] sleep
[p1] Sleeping for 4 sec
[Pipeline] [p2] echo
[p2] B <Account2>
[Pipeline] [p2] sleep
[p2] Sleeping for 2 sec
[p3] Lock acquired on [Label: automation-accounts, Quantity: 1]
[Pipeline] [p2] }
[p2] Lock released on resource [Label: automation-accounts, Quantity: 1]
[Pipeline] [p3] {
[Pipeline] [p2] // lock
[Pipeline] [p2] }
[Pipeline] [p3] echo
[p3] C null
[Pipeline] [p3] }
[p3] Lock released on resource [Label: automation-accounts, Quantity: 1]
[Pipeline] [p3] // lock
[Pipeline] [p3] }
[Pipeline] [p1] }
[p1] Lock released on resource [Label: automation-accounts, Quantity: 1]
[Pipeline] [p1] // lock
[Pipeline] [p1] }
[Pipeline] // parallel
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline

Expected Result:
I should get "C <Account2>

Perhaps I'm calling it wrong though?  It appears that the lock is being acquired BEFORE the resource is released though this might be just a logging complication of calling it in parallel