Fiber::isRunning() will only return true if you call it from within a Fiber while it is running.

<?php
$fiber = new Fiber(function ($fiber) {
    echo "Inside Fiber. isRunning: ";
    var_dump($fiber->isRunning()); // Here is true

    Fiber::suspend();

    echo "Inside Fiber again after resume(). isRunning: ";
    var_dump($fiber->isRunning()); // true again
});

echo "Before Fiber launch. isRunning: ";
var_dump($fiber->isRunning()); // Here is false

$fiber->start($fiber);

echo "After suspend. isRunning: ";
var_dump($fiber->isRunning()); // Here is false

$fiber->resume();

echo "After resume. isRunning: ";
var_dump($fiber->isRunning()); // Here is false
?>

Result:
Before Fiber launch. isRunning: bool(false)
Inside Fiber. isRunning: bool(true)
After suspend. isRunning: bool(false)
Inside Fiber again after resume(). isRunning: bool(true)
After resume. isRunning: bool(false)