Mapei Ultracolor Plus grout for Outdoor Porcelain Paving

[swaring]

I wonder if anyone could advise on the below please.
I purchased the Mapei Ultracolor Manhattan Grey grout after reading all the good reviews on the Screwfix website. I bought 3 x 5kg bags of the Mapei Ultracolor for my outdoor porcelain paving.

I mixed the product exactly as specified and with a paddle mixer. Followed the instructions EXACTLY. Mixed let it set for 3 minutes, mixed again to a nice paste. It was mixed thoroughly, no powder was left behind

A week after applying I am getting what looks like dark staining all over the grout lines.

Initially it looked great but a week on it looks terrible.

Can anyone advise on this at all?
I have attached images of the finished look.

 

[swaring]

Has anyone got any advice on this at all?

 

[Badluck12]

Has anyone got any advice on this at all?

Hi did you ever solve this? Or find out what caused it?
I have the same issue but with a different grout. Fugabella color. I would be keen to find out what you did.
Thanks