Kitten refuses to eat all but one type of food in only a few days?

I am a new cat mom and still uncertain about certain behaviors. I got 2 kittens about a month ago. I have tried to give them lots of different wet foods in brands and flavours to see what they would like and to rotate their diet so they don't get allergies and are used to diversity.

In the beginning, they ate mostly everything. Then the girl got picky about certain foods and would only eat some and not all, but her brother would happily eat up everything.

However, a few days ago, he basically started rejecting everything accept one brand, one type - Almo Nature Legend Kitten chicken. While it is good food, I don't understand why he refuses to eat every single thing I put in front of him except this in only a few days - foods he used to really like. His sister, however, is now back to eating everything - from lamb, different types of fish, raw, cooked, etc. I love it! I can put just about anything in front of her and she will eat it with gusto.

I am concerned that he will get used to only one food and then get addicted to it. Plus, I am not sure why this has happened. I read about cats getting finicky but with everything (apart from one type) in virtually a few days?

Can anyone shed a little light? Am I worrying unneccessarily?

Thanks so much.

Cats are famous for doing this.  I don't know how many times I've bought a case of food I thought mine liked only to have it sit on the shelves because they suddenly turned their spoiled little noses up to it.  On the other hand, I once had two cats who would only eat three kinds of Friskies canned food, although they were less picky about kibble.  One lived to be 14 and the other to be 19.  I don't know anything about the specific issues you raise since I am not well versed in these issues but wanted to reassure you that this is perfectly normal.