Liam, once again, tks for the reply!
After a Fenrir filled morning, I can safely say that when I use Fenrir mid battle it will land without problems, despite stunning or not on the first hit! However, when I defeat an opponent and proceed to use Fenrir on the oncoming fighter, before he hit the ground, it will consistenly whiff if the first hit stuns! And only if it stuns, otherwise Fenrir won't whiff at all. I hope this can help!
EDIT: Some players in the chat reported that the exact same happens with Excebella (use upon enemy tagging in + first hit stun leads to whiffing).