Case-insensitive file completion doesn't work with variables

[Angel-Jia]

fish version: 2.6
for example:
I have only one file named FILE_1 in the directory '~ /test/', then let dir='~ /test/', like this:
$ set dir test
then rename the file:
$ mv $dir/fi
then press tab, and no response, completion failed.
by contrast, when I am in directory '~ /test/' and rename the file
$ mv fil
then press tab, the 'fil' can be translate into 'FILE_1'