Well, to be more precise, case insensitive doesn't work with Russian letters.

So, I created white.properties file with that text:

type=item
matchItems=netherite_sword
texture=white
nbt.display.Name=iregex:abcd

And everything works great.

✅

✅

But not with Russian letters

type=item
matchItems=netherite_sword
texture=white
nbt.display.Name=iregex:абвг

✅

❌

As you can see, in the first case, everything works, but in the second, the texture does not change.
I want to note that if you change iregex to ipattern, everything will start working as it should.

Fixed by PR #139
Not approved yet because of the reasons in the issue titled CIT Resewn