“Through my most grievious fault”

Since the most recent changes to the Roman Missal almost every Sunday we’ve been saying:

through my fault, through my fault,
through my most grievous fault; …

I’ve noticed that in our congregation almost everyone says “grievious” as opposed to “grievous.” I knew that sometimes speakers insert an “-i-” into words like “grievous” and “mischievous.”  But, I didn’t realize that this was perhaps becoming the dominant way of pronouncing the word. My guess is that it’s probably on analogy with words like “devious” and “previous.” Has anyone else noticed if this is becoming the primary way that “grievous” is pronounced?  Or, is this just a quirk of our congregation?