This is not likely a bug but sort-numeric-fields function does not allow region with blank lines, while sort-lines does.
This was because sort-skip-fields occurs error with blank line. And when I added condition for sort-skip-fields like below, sort-numeric-fields was working with blank lines too.
(when (not (string-match-p "^\\s-*$" (thing-at-point 'line)))
So, I just hope sort-numeric-fields would work for region with blank lines too.
I tested with GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.18.9) of 2018-05-29
Always appreciate for contribution.