There's a lot of other theories in this thread, but the StringBuilder lock is what I can't get over from the stack traces.

  at java.lang.AbstractStringBuilder.ensureCapacityInternal(java.base@17/AbstractStringBuilder.java:228)
 - parking to wait for  <0x0000000088eee648> (a scala.concurrent.impl.ExecutionContextImpl$$anon$3)
 at java.lang.AbstractStringBuilder.append(java.base@17/AbstractStringBuilder.java:582)

That's a lock on a scala object in the middle of a java lang call-stack.

How is that lock happening?

The comments of ensureCapacityInternal explicitly says

    /**
     * This method has the same contract as ensureCapacity, but is
     * never synchronized.
     */
    private void ensureCapacityInternal(int minimumCapacity) {