It would appear that Elemental Drain is currently suffering from a minor bug related to resource drain rather than recovery when characters proccing it have a disparity in maximum Magicka between their two skill bars.
In order to make this bug happen, you need to have a character setup in a way that creates the disparity between maximum Magicka on your two skill bars, which can be achieved by using class skills on some classes or Inner Light (or any Mages Guild ability provided you put at least 1 point in the "Magicka Controller" passive from that skill tree).
I have recorded a very short testing session which is meant to show exactly what this bug causes:
https://www.youtube.com/watch?v=S7SvH8ZlXlg
What I need the reader to notice is how after I create the disparity in maximum Magicka by removing Inner Light from my backbar, either the first or second damage tick (from any source, have tested all class DoTs, light and heavy attacks, direct damage skills) will drain a certain amount of Magicka. The exact quantity seems to be affected by maximum Magicka and/or the amount of damage inflicted to proc drain (unclear if either or both are factors). The bug will only occur once every time you swap to the skill bar with less maximum Magicka.
Seeing as Elemental Drain according to its description is meant to restore Magicka, this effect is self-evidently unintended.

Shoutout to @Ceptor131 for bringing this to my and other guildies' attention.