Legion able to hit survivors being unhooked with Feral Frenzy to get free chain
Just played a Legion game with Joey's Mixtape and Defaced Smiley Pin.
Survivor 1 (Kate) and Survivor 2 (Feng) were approaching the hooked Survivor 3 (Jeff). Used Feral Frenzy to damage Kate and Feng on their way to the hook (2 hits). Feng proceeded to unhook Jeff in front of me and I got a heartbeat when the UNHOOKING ANIMATION WAS HAPPENING, so I just went with a hit before he touched the floor, which didn't interrupt Feral Frenzy, but instead it counted as a hit (3 hits). Feng had Borrowed Time. Since Jeff was mid unhook animation, the 3rd hit DID NOT PROC DEEP WOUND, so Jeff still had a heartbeat after being unhooked. Hit him again (4 hits), which activated BT's protection (he got the white-ish aura), but was able to insta-down him again with the 5th hit.
The map was Azarov's Resting Place.
The hook in question was a regular hook
I doubt hitting a survivor while being unhooked is intended to count towards Feral Frenzy stacks.

Comments
I can provide that video.It is a free chain from the time it is unhooked until the nair is ready to move.
https://twitter.com/i/status/1520219261779988480This has been acknowledged.
https://forum.deadbydaylight.com/en/discussion/320877/feral-frenzy-hits-in-survivors-being-unhook-counting-towards-the-5-hits-to-down-a-survivor