Your Mgmt is at its max Log receiving capacity when the fw_full is consistently close to a 100% (80-90% is close).
When you have log traffic peaks that cause it to reach ~100%, it may stop receiving logs & the GW will locally logs on itself for a few secs or more, depending on load.
Unless the log-rate is actually low, cause you said the fw_worker's CPU is low, then it requires a TAC investigation.
Can you specific Mgmt HW details & Log-rate?
run on Mgmt: cpstat mg -f log_server
If possible, I would advise to up the Mgmt's HW specs (CPU mostly).