The passive SOMETIMES, on some toons, gives the 8% magicka on the wrong hotbar.
Both my stamina nightblades are affected by the bug on live. If I respec, the bug will disappear for a little, then come back (after relog maybe?)
Strangely I have never had any issues with the passive on my magicka nightblade. But the moment I respecced it to be stamina build and relogged, the issue showed up.

In the video I link below, I have no siphoning ability on bar 1, and a siphoning ability on bar 2. and I get the extra magicka from siphoning ability on bar 2 as intended. But as soon as I relog (second part), the bug comes back: I have no siphoning ability on bar 1, and a siphoning ability on bar 2. But I get the extra magicka from siphoning ability on bar 1.


The bug does not appear right after a login or respec. But it seems if you relog after that, it returns.

I also tested how many elemental blockade I could cast, and it is clearly not a UI bug. I can cast 1 more elemental blockade on the front bar. I made sure to equip ice staff and hold block to avoid any magicka regen.

Tried replicating the bug on pts and I couldn't at first. But I then did 2 things, and one of them activated the bug: I put 64 points in stamina stat. And I equipped mechanical acuity set (but 4 piece stat bonus only active on one hotbar). I believe it's related to either mechanical acuity, or possibly stats from sets not active on both bars.

Edit: Tested with necropotence 4 piece only active on one hotbar. And by slotting it on same bar as necropotence I actually managed to have magicka flood active on both bars. But if I slotted it on the non necropotence bar, it did not work at all on either bar. (did not have pet active so 5 set bonus not active)
Edit: tested with necropotence 3+4 piece only active on one hotbar. This made it work like mechanical acuity, so only active on the wrong bar.
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Link to video of test:
https://youtu.be/-F2ZEbtNbuQ