There were some bugs due to which FastTrack counters increased despite going the slow path. Moreover, FastTrack counters sometimes may “lie”. A classic setup example: vlan-filtered bridge for LAN network + a standalone upstream port to ISP. When a packet is received on the upstream port, it is eligible for FastTrack, and therefore increasing counters. However, later in the chain, the packet enters the bridge, which cannot do FastPath due to VLAN filtering, redirecting it via the slow path. In the end, the packet has traveled through the slow path while still increasing FastTrack counters.
We have fixed a few counter-related issues in the upcoming release. However, solving all of them would require significant performance overhead, which is unacceptable for FastPath/FastTrack.