[2.6] Siphon strike does not work with the powerful Sledgehammer blow

[Sawyer]

Version

2.6

Platform

Windows

Hello,

I'm playing version 2.6 on PC, and using the Sledgehammer.
I tried the "Siphon Strike" perk, with only 1/5 of a point invested, but it only works with the normal attack (left click).

In other words, I only regenerate my health when I kill zombies with a normal headshot; it doesn't work with the heavy attack. I've tried it several times : I killed hundreds of them by blowing their heads off with a powerful blow, and the zombies didn't die from collateral damage (bleeding or fire), but it only regenerate me with the normal blow.

Is it normal that it only works with the normal attack and not the heavy attack? Or is it a bug that needs a patch?

I ask because I prefer to use only the heavy attack (right click) as it deals more damage (it almost always kills zombies instantly) and fully regenerates my stamina thanks to the special perk (I've read all 7 books).

Thank you.