While playing around I noticed two additional cases where GNU printf pads with zeros whereas uutils printf doesn't:

$ /usr/bin/printf ">%#15.6o<\n" "0"
>         000000<
$ cargo run printf ">%#15.6o<\n" "0"
>         0     <

$ /usr/bin/printf ">%#15.6o<\n" "08"
>/usr/bin/printf: ‘08’: value not completely converted
         000000<
$ cargo run printf ">%#15.6o<\n" "08"
printf: '08': value not completely converted
>         0     <

I think that's just one case right? The second case just parses the zero and then exits. The difference in spacing comes from how the error message is printed. Good catch, though. I'll try to get that edge case in.

Ah hold on actually. 0 values for other formats get padded too.