Using Arcane Cloak inside Mana Siphoner removes all mana regen even outside the circle.

A clip of what I mean:
https://www.youtube.com/watch?v=Y8ku1LGaWYM

I used Arcane Cloak that is linked to Arcane Surge inside Mana Siphoner, and lost all mana regen afterwards until I attempted to use Arcane Cloak again at 0 mana. Not entirely sure if that interaction is the actual cause of the loss of mana regen, but I can't see anything else that would have caused it.

Your character is using Mind Over Matter, The Agnostic, and Righteous Fire.

There's currently an issue with combining RF with those defensive layers, where if your mana hits 0, it won't increase past 0 unless you get flat recovery, or your regen can fully cover both the drain from Mind Over Matter and the maximum possible drain from The Agnostic--which, obviously, is a lot.

The reason you were able to regen mana after casting Arcane Cloak again was due to your mastery that recovers 10% of mana over 1 second after using a Guard skill.