IoT's RegisterThing intermittently not working

I successfully use CreateKeysAndCertificate via an MQTT topic (IoT provision by claim). But then when I use RegisterThing via an MQTT topic, it only works about 50% of the time, even though my IoT code and AWS settings remain unchanged.

My pre-provisioning hook Lambda is called every time by RegisterThing according to the CloudWatch logs. And the Lambda simply returns allowProvisioning : true every time within 200 ms according to CloudWatch. But HALF the time nothing ever comes back on $aws/provisioning- templates/myThing/provision/json/rejected nor $aws/provisioning- templates/myThing/provision/json/accepted. RegisterThing is kind of a black box in the cloud, so is there something I can do?

Edit: It seems that when the previous call to RegisterThing (via MQTT) was made longer than ~ 5 minutes ago, a call to RegisterThing fails to publish its success to the $aws/provisioning-templates/myThing/provision/json/accepted topic.

I want $100 or a job offer for rigorously experimenting to discover this rather obscure issue... Ha! But seriously, I'd appreciate if someone else can confirm on their end what I'm seeing.