Master Assassin, NB passive.

Snamyap

I was comparing two characters, a Nightblade and a Templar in pretty much the same gear and exact same champion points set up and couldn't figure out why their penetration stats were different. Turns out the penetration bonus from Master Assassin, which (should) only work when flanking, shows up as standard penetration on the character sheet. Should that be happening?

Edit: Tested on a dummy, and the passive works as intended. But with the character sheet misleading me I was actually under the penetration threshold on my Nightblade, which is annoying since I like to solo dungeons and rarely end up flanking mobs there.

Edit 2: Turns out the info on the character sheet automatically changes depending on whether you stand in front or behind a target. Confusing.

Lykeion

This has to do with the orientation that your character (not the camera) is facing. I posted a bug report about this situation a long time ago, but I haven't gotten a response from the dev until now. It's probably going to be around for a lot longer

https://forums.elderscrollsonline.com/en/discussion/605395/nbs-master-assassin-not-triggered-by-flanking-but-what-cardinal-direction-you-are-facing

Snamyap

Heh, looks like you're right. My dummy faces more or less south. When I stand facing it and press c my character turns south and I loose the bonus. Standing in front of it with my back turned towards it my character turns north when pressing c and gains the bonus.
This has got to be the most ridiculous bug I've encountered in ESO. 🤦

P.s. Makes me wonder if the Flanking Strategist and Dragonguard Elite sets work properly.