Hi Mark,
I have seen it happening if it's not configured properly, I would recommend to keep it off if not using. Please follow the below KB and control the bandwidth on the correct ingress/egress ports.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB25916&cat=NS_25&actp=LIST
Some helpful commands to check the status during the speed tests:
get traffic-shaping interface
get traffic-shaping statistics
get traffic-shaping mode
Thanks,
Vikas