Omen of sinistral necro

I just had an omen give me a sufix instead of the guarantee prefix on some boots after using this omen revealing on boots for 14 cold resist. Im 100% sure I had the omen on because it was still on even after revealing. Boots had 2 prefix and 2 suffix. Not sure if its has a small chance to fail or something, just wanted to bring it to you attention.

thanks

I was able to recreate it by using the rib bone first then moving the omen to my inventory, which is what i must of done last time. I misunderstood and thought it only had to be in inventory before going to abyss pit to reveal and not before you use the one the bones in the first place.

Thanks.