File content lost on editing a hardlink

On editing two files which are hard links, the last saved one is empty.
Reproduction:
$ echo hello > foo.txt
$ open foo.txt # with textastatic
$ ln foo.txt bar.txt
$ cat bar.txt
hello
$ open bar.txt # with textastic
...add content to the file and save or just save...
$ ls -al
-rw-r--r-- 1 user wheel 0 14 mar 12:05 bar.txt # <- now empty!
-rw-r--r--@ 1 user staff 7 14 mar 12:05 foo.txt