That should not be the reason. Arm displays the flags right in the text gui when used without parameters. Only when called via "-p" for command line processing it seems to fail.
That does seem a tad odd though. For what it's worth though the interpreter's been moved out of nyx (aka arm) and into stem. Stem's interpreter got quite a few improvements...
https://stem.torproject.org/tutorials/down_the_rabbit_hole.html
Cheers! -Damian