Why does bash give the following result after brace expansion?

I am using linux and the following version of the bash:

GNU bash, version 5.1.16(1)-release (x86_64-pc-linux-gnu)

When I type:

echo file{[1,2],3}.txt

I expect brace expansion to be done first, so:

echo file[1,2].txt file3.txt

After that, I expect it to stop there, since [1,2] is not a valid globbing. However, it gives the following as output:

file[1.txt file2].txt file3.txt

Why? List of files in current directory (result of ls command):

go.mod  hello.sh  hi  main.go