wrong --prefix, expecting 1.1.1.tar.gz:

$ pijul init p1
$ cd p1
$ touch a
$ pijul add a
$ pijul rec -am.
$ pijul archive -o 1.1.1 --prefix 1.1.1
$ ls -l
total 4
-rw-r--r-- 1 mpech mpech 77 jūl  2 17:04 1.1.tar.gz
-rw-r--r-- 1 mpech mpech  0 jūl  2 17:01 a
$

I tried your test and the prefix is correct (folder inside the archive), but the output name is incorrect.

I had previously tried pijul archive -o ~/test.zip and I got a ~/test.tar.gz output file.