That is indeed odd, while I am not 100% certain it worked in 1.26 I am pretty certain it did.
I suppose that means it is not related to the patch bur rather your computer, I suppose you have tried re-extracting JNGP?
edit: it might be a permission thing, in order to mofidy the tileset you need to have permission to modify the map. Which is not allowed in certain folders and whatnot.