I am currently migrating jenkins between machines, and i can confirm that in 1.537 with throttle 1.8 (old machine) everything is working fine, and with 1.538 with throttle 1.8 (new machine) it is broken, it doesn't work at all.
(edit) Aparently the problem is with the labels, removing the labels did make everything start to work again (in the old machine i had restriction by node name, in the new i was trying to restrict by label). changing restriction to node name only in the new machine didn't work, i had to remove labels from the nodes as well.
Other thing i found when restricting by label. I created a job with 10 downstream jobs all restricted by the same category (configured to restrict once across all nodes). When i run the job, it start working fine, with the "pending, one job already running" message. if i canceled the job running, then remaining jobs started running in all possible executors, and the message for the ones still in the queue (there were no more available executors) changed to "pending, 5 jobs already running"
upgrading jenkins to 1.542, the problem persists with or without labels.
I can confirm that builds were successful for us until early this week where we upgraded to Jenkins.
Throttle 1.8 and Jenkins 1.536