That is true. ROMON frames are not forwarded by UNIFI.
In terms of your magical "-200% packet loss" i have really simple explanation:
You are pinging MAC address. Since you did not specify which interface you want to transmit, it will transmit on ALL interfaces. including bridged ethernets or vlans. (therefore one request gets actually transmitted multiple times)
In addition, anything what has this address will respond. If there are multiple interfaces or devices with the same mac address, they will respond at the same time.
In your case, it is clear that each Seq # is repeated 3 times. That means 3 replies are received per each request.
0 reply per each request = 100% loss
1 reply per each request = 0% loss
2 replies per each request = -100% loss
3 replies per each request = -200% loss
I know the number like this does not make sense, however, there is no other way to summarize packet loss based on amount of transmited requests and received replies.
I tested the same on couple of routers which all had Ether1 in Bridge1 (no VLANs/VRRP etc..) and all interfaces had different MAC. Therefore I had guarantee, that there is only one interface with destination MAC available. When I sent the request without specified source interface, I always received 2 replies. When I specified source interface, I always got only one reply (or none, if I intentionally selected interface which was not connected)
Due to that, I am sure that in my example, request was transmitted by Ether and also by Bridge, which was connected to Ether, so there were actually 2 request frames sent via Ether...