Read this please.
The notion of Regulus having Acausality comes from the following scans, specifically the scans to do with Laws/Rules: There was only one possibility left of all the patterns he had imagined that could explaināhis power didnāt make him invincible at all. It was a time stop ability. More...
vsbattles.com
TLDR: So basically, every time the text talks about Regulus being unaffected by shit, is only refer to physical things, and he never show on the story to not get affected by Causality Manipulation, Fate Manipulation, and Precognition. He actually have anti-feats for that.
The thread above explain a little better, so just read that.