Device not outputting HTTP Header error into syslog?

You can list db keys with

tmsh list sys db all-properties

Then use diff or similar to compare the output between devices.

There may just be an edge case where the violation is triggered but no message is emitted - maybe due to incoming packet segmentation. The HTTP profile can be like that sometimes.

If you are concerned, raise a support ticket - there may be stats available in the QKView that give some visibility to the problem. A nice clean packet capture might be a good idea as well.