Invisibility broken at random when preparing a spell

[TenebrisFractum]

Usually when I was invisible I could charge up a destruction spell and go out of invis as I release it. Now for some reason that doesnt work sometimes. Instead I randomly break invisibilty when preparing myself which is kinda annoying and can lead to dumb situations. With the spell Chaos for example I would usually go invis, run up to a group of enemies, charge up, release and cast invis again to disappear and let them kill each other.

That randomly fails for some reason.

Any ideas why?

 

I use mods like Ordinator and Apocalypse (patched) but they seemed to work properly so far so Im wondering what causes this.