This is correct, Mana Regen is set to 0, even from other items. You can still gain mana through items and abilities that directly give it though. Like Energy banner says all adjacent monsters gain 2 mana, that is unaffected because it is not regen.
Ahhh now I understand. Mana regen and mana "gain" are two separate stacks. I see.
Is there a way to see how much mana regen a monster have? I still don't understand if there is. It could be handy to calculate how much time is needed to cast, sometimes, especially on calamity mode, where one mistake is fatal.
That's good to know! Can I suggest to put these infos in the finished/polished game? :D These things are great to know. It's not a must, but I always find weird to look these things up online for various strategic games, even AAA ones. Sometimes they don't show some data and I don't quite get why there is not a menu that shows them, somewhere. You can play without them, for sure, but why keeping them hidden? o.o