UCG Fiber - high CPU usage

It's been some month I have due UCG Fiber in HA setup (shadow mode), no problem so far except that this is the second time in about 10 days that the CPU goes crazy:

https://preview.redd.it/2vs36ro4c9ch1.png?width=766&format=png&auto=webp&s=e93ddcb7e95c05f2bd6bb0ea7ce3005aa1907749

10 days ago, on first occurrence, I logged in via SSH and restarted the control plane (unifi) service and everything was back to normal and now the same thing.

Unifi OS 5.1.19

Network 10.4.57

We have some traffic but I would not say crazy traffic

https://preview.redd.it/rq037riyc9ch1.png?width=3316&format=png&auto=webp&s=090b0772160a49491e889c6d05737f6b77b534e1

with about 45-55 clients where mostly virtual servers and some public exposed (web) service.

Already tried to enable/disable IPS, traffic identification, ad block, etc: no difference at all. The only way to decrease the CPU usage is restart the unifi service.

Do you have any idea of what the root cause could be?